UC-NRLF 


iao 


AND 


OF  COLLOIDS 


HATSCHEK 


FOURTH  EDITH 


Botany- Plant   Ph 


TEXT-BOOKS  OF  CHEMICAL  RF^EARL^H'  AND'  ENGINEERING 

AN  INTRODUCTION  TO 
THE  PHYSICS  AND 
CHEMISTRY  OF  COLLOIDS 


BY 

EMIL    HATSCHEK 


Fourth  Edition 

Entirely  Re-written  and  Enlarged 
With  20  Illustrations 


PHILADELPHIA 

P.   BLAKISTON'S  SON   &   CO. 

1012  WALNUT  STREET 
1922 


Printed  in  Great  ttvitain. 


dot 


PREFACE  TO   FOURTH    EDITION 


THE  present  edition  has  been  entirely  rewritten 
and,  incidentally,  enlarged.  The  changes  made  are 
partly  the  outcome  of  increased  experience  in  teach- 
ing the  subject,  and  partly  caused  by  the  inclusion 
of  important  recent  work  on  fundamental  aspects 
of  the  science.  The  literature  up  to  the  middle  of 
the  present  year  has  been  drawn  upon,  as  far  as  it 
falls  within  the  purview  of  the  present  book  ; 
references  are  given  to  such  papers  as  have  not 
found  their  way  into  the  majority  of  text-books.  At 
the  same  time  the  author  has  endeavoured  to 
preserve  the  general  character  of  the  work. 

As  Laboratory  Manuals  of  Colloid  Chemistry  are 
now  available,  the  short  appendix  on  experimental 
technique  added  to  the  third  edition  has  been 
omitted. 

EMIL   HATSCHEK 

LONDON,  October^  1921. 


482156 


PREFACE    TO   FIRST   EDITION. 


THE  present  work  is  a  slightly  enlarged  reprint 
of  a  series  of  articles  published  in  The  Chemical 
World,  which  in  their  turn  were  based  on  a  course  of 
ten  lectures  delivered  at  the  Sir  John  Cass  Technical 
Institute  to  students  of  very  varied  attainments 
and  interested  in  every  branch  of  chemistry  and  of 
chemical  industry.  The  book  accordingly  does  not 
aim  at  a  completeness  precluded  alike  by  its  compass 
and  the  extremely  vigorous  growth  of  the  subject, 
but  is  only  intended  to  introduce  readers  with  a 
reasonable  knowledge  of  physics  and  chemistry  to 
the  fundamental  facts  and  methods  of  a  branch  of 
physical  chemistry  on  the  importance  of  which  it 
is  hardly  necessary  to  insist. 

Certain  features  in  the  selection  of  subjects  and 
in  the  order  of  presenting  them,  which  will  be 
apparent  to  readers  familiar  with  the  existing 
literature,  are  due  to  a  vivid  recollection  of  the 
difficulties  experienced  by  the  author  in  his  first 
studies,  and  the  desire  to  spare  the  student  as  many 
of  these  as  appear  avoidable.  Those  desirous  of 
ampler  and  more  detailed  information  are  referred 
to  Wolfgang  Ostwald's  "  Grundriss  der  Kolloid- 
chemie  "  and  H.  Freundlich's  "  Kapillarchemie," 
English  translations  of  which  are  urgently  required. 

EMIL   HATSCHEK. 

LONDON,  1913. 


CONTENTS 

PAGE 

PREFACE  TO  FOURTH  EDITION.          .          .          .          .       v 

PREFACE  TO  FIRST  EDITION      .          .          .          .          .      vi 

CHAPTER  I.      .          .          .          .          .          .          .  i 

History  of  subject.  The  work  of  Thomas  Graham. 
Earlier  investigators.  Modern  development. 
Generality  of  the  colloidal  state.  Artificially 
prepared  inorganic  and  natural  organic  colloids. 
The  disperse  systems.  Possible  types  and  classi- 
fication according  to  the  state  of  aggregation. 
Specific  surface.  Simple  experiments. 

CHAPTER  II.    .          .          .          .          .          .          .          .13 

Methods  of  investigation  as  applied  to  elucidation 
of  peculiarities  of  colloidal  state.  Dialysis,  filtra- 
tion and  ultra-filtration.  Sizes  of  pores  in  ultra- 
filters.  Tyndall  cone,  size  of  particles  as  compared 
with  wave-length  of  light.  Limits  of  microscopic 
visibility.  Slit  ultra-microscope  and  calculation  of 
size  of  particles.  Ultra-  and  dark  ground  con- 
densers. 

CHAPTER  III.  .          .          .          .          .          .          .     25 

Behaviour  of  sols  to  electrolytes.  Lyophile  and 
lyophobe  colloids.  Other  grounds  of  classification. 
Viscosity  of  liquids.  Striking  difference  between 
two  types  of  colloids  in  respect  of  viscosity. 
Reasoning  by  analogy  from  coarser  systems. 
X-ray  analysis  of  gold  gels.  Solvation.  Anomalies 


viii  CONTENTS. 

PAGE 

and  temperature  coefficient  of  viscosity.  Segre- 
gation of  sols  into  two  liquid  layers.  Classification 
into  suspensoids  and  emulsoids  adopted. 

CHAPTER  IV.  ........      31 

Stokes's  formula.  Discussion  and  numerical 
example.  Small  size  alone  inadequate  to  explain  - 
properties  of  sols.  Brownian  movement.  Results 
of  experimental  and  mathematical  investigation. 
Perrin's  work.  The  electric  charge  and  its  influence 
on  stability.  The  iso-electric  point.  Demonstra- 
tion of  charge  by  the  U-tube  and  the  microscopic 
method. 

CHAPTER  V.    .          .          .          .          .          .          .          .40 

Suspensoid  sols.  Dispersion  and  condensation 
methods  of  preparation.  Examples  of  the  latter  : 
sols  of  the  noble  metals,  selenium,  sulphur.  Sul- 
phide and  halide  sols.  V.  Weimarn's  theory  of  the 
condensation  methods.  Dispersion  methods.  Pep- 
tization.  Grinding  with  and  without  chemical 
treatment.  Electric  disintegration.  The  work  of 
Bredig  and  Svedberg. 

CHAPTER  VI 48 

General  properties  of  suspensoid  sols.  Viscosity. 
The  viscosity-concentration  curve  ;  influence  of 
dispersity  and  rate  of  shear.  Einstein's  formula. 
Discrepancies.  Hess 's  formula.  Optical  properties. 
Tyndall  cone  and  its  state  of  polarization.  Colour 
of  sols.  Gold  sols.  Silver  sols. 

CHAPTER  VII.  .......     54 

Electrolyte  coagulation.  General  character  of 
phenomenon.  Effect  of  valency  of  coagulating 
ion.  The  Schulze-Hardy-Whetham  rule.  Inade- 


CONTENTS.  ix 

PAGE 

quacy  of  simple  valency  rules.  Specific  nature  of 
H  and  of  organic  ions.  Effect  of  anion.  Influence 
of  concentration  and  dispersity  of  sol  particles. 
Positive  sols.  Coagulation  velocity. 

CHAPTER  VIII 62 

Mutual  coagulation  of  sols  with  opposite  charges. 
Protective  colloids.  The  gold  number.  Reactions 
in  presence  of  protective  colloids.  Theory  of 
protective  action.  Protection  after  coagulation. 

CHAPTER  IX.  .......     66 

The  hydroxide  sols.  Reasons  for  separate  treat- 
ment. Hydroxides  of  Al,  Fe  and  Cr.  Stannic  acid 
and  the  purple  of  Cassius.  Hydrolysis  methods. 
Ceric  hydroxide  sol.  Variety  of  behaviour  towards 
electrolytes. 

CHAPTER  X.    .          .          .          .          .          .          .          -70 

Coarse  systems  of  two  liquid  phases.  Emulsions. 
The  pure  oil-water  emulsions.  Behaviour  towards 
electrolytes.  Surface  rigidity  of  liquid  globules. 
Phase  ratio  unlimited.  Emulsifying  agents  and 
their  general  characteristic  :  reduction  of  inter- 
facial  tension.  The  drop  pipette.  Probable 
existence  of  interfacial  film.  Viscosity.  Analogies 
applicable  to  emulsoid  sols. 

CHAPTER  XI.  .......      78 

Silicic  acid.  The  work  of  Graham.  Effect  of 
electrolytes.  Importance  of  anion.  Ceric  hydroxide 
sol.  Tungstic  acid.  Variety  of  character. 

CHAPTER  XII 81 

Organic  emulsoids.  Gelatin  and  agar.  The  sol-gel 
transformation.  Melting  and  setting  point  of 
gelatin.  Hysteresis  and  ageing.  Rise  of  viscosity 


x  CONTENTS. 

PAGE 

with  age.  Action  of  electrolytes  very  complicated. 
Precipitation  and  segregation  into  two  liquid 
layers.  Effect  of  neutral  salts  on  setting  point  and 
setting  velocity.  Preponderance  of  anion.  Re- 
fractive index  and  rotation.  Transformation  into 
gelatose. 

CHAPTER  XIII .86 

Albumin.  Preparation  of  pure  albumin.  The 
heat  coagulation.  The  effect  of  neutral  salts. 
Effect  of  cation.  Effect  of  anion  and  the  Hof- 
meister  series.  Importance  of  the  latter  in  other 
phenomena.  Lyotropy.  Neutral  albumin  and 
albumin  salts  with  acids  and  bases.  Hydration  and 
degree  of  ionization.  Globulins.  Casein. 

CHAPTER  XIV.         .......      92 

Other  aqueous  emulsoids.  Starch,  dextrin,  gum 
arabic.  Cellulose.  Non-aqueous  emulsoid  sols. 
Cellulose  esters,  india-rubber.  Connexion  between 
viscosity  of  sols  and  physical  properties  of  dry 
substance.  Dialysis  of  non-aqueous  sols. 

CHAPTER  XV.  .......     95 

General  properties  of  emulsoid  sols.  Optical 
properties  and  ultra-microscopic  appearance.  Vis- 
cosity. Increase  with  concentration.  Temperature 
coefficient.  Anomalies.  Influence  of  rate  of  shear. 
Osmotic  pressure  of  emulsoids.  Ageing  and  its 
effect  on  viscosity. 

CHAPTER  XVI.         .          .          .          .          .          .          .    101 

The  soaps.      Conductivity   and   osmotic   pressure.     » 
The  colloidal  ion.      The   dyes.      Congo   Red  and 
Congo  Rubin.     Other  dyes.     Mutual  precipitation 
of  dyes.     Semi-colloids. 


CONTENTS.  xi 

PAGE 

CHAPTER  XVII.       .......    105 

Gels.  Elastic  and  rigid  gels.  Silicic  acid  gel. 
Formation  and  syneresis.  Drying  of  gel  and 
alterations  caused  thereby.  The  drying  curve. 
The  turning  point.  Structure  of  the  dry  gel. 
Ceric  hydroxide  gel.  Tanned  gelatin  gels. 

CHAPTER  XVIII.      .          .          .          .          .          .          .in 

Elastic  gels.  Gelatin.  Swelling.  Volume  changes 
and  liberation  of  heat.  The  cedometer.  Altera- 
tions in  shape  during  swelling  and  drying.  Effect 
of  solutes  on  swelling  ;  the  lyotropic  series. 
Elastic  modulus.  Accidental  double  refraction 
caused  by  strain.  Syneresis  in  clastic  gels. 

CHAPTER  XIX.         .          .          .          .          .          .          .118 

Diffusion  in  gels.  Effect  of  solutes  on  rate  of 
diffusion.  Experimental  difficulties.  Reactions 
in  gels.  Difference  between  soluble  and  insoluble  . 
reaction  products.  Pringsheim's  rule.  Formation 
of  large  crystals.  The  Liesegang  phenomenon. 
Theory  of  the  latter. 

CHAPTER  XX.  .......    125 

The  structure  of  gels.  Dry  gels.  Rival  theories 
of  gel  structure.  Evidence  for  heterogeneous 
nature.  State  of  aggregation  of  phases.  The  work 
of  Buetschli  and  ultra-microscopic  evidence.  The 
micellar  structure.  Gels  from  well-defined  crystal- 
line substances.  Review  of  theories. 

CHAPTER  XXI.         .          .          .          .          .          .          .131 

Adsorption.  Familiar  examples.  Surface  energy 
and  possibilities  of  reduction.  The  surface  solid- 
liquid.  Adsorption  and  surface  tension  at  boun- 
dary mercury-gas.  Excess  concentration  in  froths. 


xii  CONTENTS. 

PAGE 

The  Gibbs  theorem.  Discussion.  Anomalous 
cases,  adsorption  of  sugar  by  charcoal. 

CHAPTER  XXII.       .          .          .          .          .          .          .138 

Further  conclusions  from  Gibbs 's  formula.  Ad- 
sorption from  mixed  solutions.  Displacement  of 
adsorbed  substance  by  another.  The  effect  of 
surface.  Wislicenus's  alumina.  The  adsorption 
equilibrium.  Ostwald's  and  Freundlich's  experi- 
ments. Time  required  to  attain  equilibrium. 
Anomalous  cases.  The  concentration  function. 
Values  of  exponent.  Effect  of  adsorbent  and  of 
solvent.  Adsorption  curves.  Anomalous  cases. 
Adsorption  of  electrolytes.  Adsorption  followed 
by  chemical  action. 

CHAPTER  XXIII.     .......    148 

Adsorption  from  mixtures.  Capillary  analysis. 
Kraus's  modification  of  the  latter.  Possibility  of 
adsorption  in  presence  of  finely-divided  phases  and 
criteria  of  process.  Extraction  of  adsorbed  sub- 
stances. The  extraction  curve.  Electrical  adsorp- 
tion. General  importance  of  adsorption. 

CHAPTER    XXIV.     .......    154 

Theories  of  the  origin  of  the  electric  charge  on 
disperse  particles.  The  double  layer.  Coehn's 
hypothesis.  Difficulties  regarding  dielectric  con- 
stant. Sols  of  the  same  substance  with  opposite 
charges.  The  adsorption  of  ions.  Freundlich's 
theory  of  electrolyte  coagulation.  Objections  to 
adsorption  theory.  The  colloidal  electrolytes, 
especially  soaps  and  proteins.  Pauli's  general 
complex  theory  of  electric  charge. 


CONTENTS.  xiii 

PAGE 

CHAPTER  XXV.        .......    161 

Applications.  Theoretical  :  problems  of  physi- 
ology. Adsorption  :  problems  of  soils  ;  the 
photohaloids  and  the  latent  image.  Technical  : 
empirical  nature  and  age  of  industries  using  colloid 
material.  Variety  of  problems.  The  study  of 
emulsions.  The  application  of  viscosity  measure- 
ments in  various  directions.  Swelling  in  its 
technical  aspect.  Tanning.  Effects  of  degree  of 
dispersity.  The  ripening  of  photographic  emul- 
sions. Dispersity  and  colour,  glass,  artificial 
ultramarine.  Precipitates.  Technical  methods 
involving  the  electric  charge.  Electro-endosmosis 
and  cataphoresis  on  technical  scale.  Conclusion. 


SUBJECT-MATTER  INDEX    ......    166 

NAME  INDEX    .          .          .          .          .          .          .          .170 


WORK  OF  THOMAS  GRAHAM. 

substances  generally  known  as  insoluble  could,  by 
appropriate  methods,  be  obtained  in  what  at  first 
sight  appeared  to  be  real  solutions.  These  methods 
will  be  discussed  in  detail  later  on,  but  it  may  be 
mentioned  here  that  Graham  obtained,  and  very 
carefully  investigated,  such  solutions  of  silicic  acid, 
tungstic  acid  and  a  number  of  basic  hydroxides,  like 
those  of  iron,  chromium  and  aluminium.  Most  of 
these  appeared  to  the  eye  like  true  solutions,  i.e.,  they 
were  perfectly  clear  and  apparently  homogeneous. 
They  did,  however,  not  diffuse  through  parchment, 
and  on  account  of  this  property  were  called  by 
Graham  "  colloidal  solutions/'  or  simply  "  sols," 
which  term  has  now  become  generally  accepted. 
These  sols  showed  a  further  striking  peculiarity, 
which  distinguished  them  sharply  from  true  solutions 
such  as,  say,  cane  sugar  or  sodium  chloride  solutions, 
inasmuch  as  very  small  additions  of  electrolytes, 
which  did  not  react  at  all  with  the  dissolved  substance, 
caused  radical  alterations  in  the  condition  of  the  sols. 
Traces  of  carbon  dioxide,  for  instance,  caused  the 
silicic  acid  sol  to  set  to  a  translucent  jelly,  while  small 
additions  of  any  neutral  salt  would  precipitate  the 
ferric  hydroxide  as  a  flocculent  mass.  These  trans- 
formation products  of  sols  were  called  "  gels  "  by 
Graham,  a  title  still  in  use,  though  generally  employee! 
in  a  restricted  sense. 

While  Graham's  investigations  were  the  first  ones 
directed  systematically  towards  the  production  of  sols 
and  the  application  to  them  of  dialysis,  both  as  a 
means  of  preparation  and  as  a  test,  numerous  earlier 
observations  of  sol  formation,  i.e.,  the  production  of 
apparent  solutions  of  substances  known  as  insoluble, 
are  not  wanting  even  in  the  early  days  of  chemistry. 
Between  1808  and  1833  Berzelius  had  observed  what 
would  now  be  called  sols  of  sulphur  and  arsenious 
sulphide,  of  "  b-  "  silicic  acid,  tellurium,  etc.  Sulphur 
sols,  obtained  by  the  reaction  between  hydrogen 


HISTORY   OF  METAL  SOLS.  3 

sulphide  and  sulphur  dioxide  in  aqueous  solution, 
were  investigated  carefully  by  Wackenroder  (1846), 
and  by  Selmi  and  Sobrero  (1850).  They  were  again 
studied  by  Debus  (1888),  who  remarked  on  the 
similarity  between  them  and  the  sols  of  Thomas 
Graham. 

Of  particular  interest,  in  view  of  the  great  theore- 
tical importance  which  they  ultimately  acquired,  is 
the  history  of  the  metal  sols.  Red  or  purple  liquids, 
obtained  by  the  reduction  of  gold  chloride,  were 
known  to  the  alchemists  and  were  used  medicinally 
under  the  title  of  "  auruni  potabile."  Various  direc- 
tions for  making  the  latter  are  given  in  Marcquer's 
"  Dictionnaire  de  Chymie  "  as  late  as  1774.  The 
earliest  reference  in  English  literature  is  probably  a 
somewhat  tantalizing  entry  in  the  Diary  of  John 
Evelyn,  who  writes  on  June  27th,  1653  :  "  Monsieur 
Roupel  sent  me  a  small  phial  of  his  aurum  potabile, 
with  a  letter  showing  the  way  of  administering  it, 
and  the  stupendous  cures  it  had  don  at  Paris  ;  but 
ere  it  came  to  me,  by  what  accident  I  know  not,  it 
had  all  run  out." 

In  1839  Wohler  found  that  silver  citrate  heated 
in  a  stream  of  hydrogen  left  a  residue  which  dissolved 
in  water  with  a  red  colour.  The  experiment  was 
repeated  and  frequently  referred  to  in  the  dispute 
regarding  the  existence  of  silver  "  sub  "-salts,  as  which 
Wohler  had  regarded  the  substance  obtained  by  him. 
In  1887  Muthmann  repeated  the  experiment,  dialyzed 
the  red  solution  and  showed  that  the  red  substance 
did  not  pass  through  the  membrane,  but  that  only 
undecomposed  silver  citrate  could  be  found  in  the 
outside  water.  This  investigation  is  of  great  interest 
as  being  the  first  one  in  which  dialysis  was  applied  to 
a  metal  sol.  Carey  Lea  (1889)  developed  various 
methods  for  the  reduction  of  silver  which  yielded 
sols  of  high  concentration  ;  analysis  showed  the 
coagulum  from  these  sols  to  be  silver  with  a  consi- 

1—2 


4     FARADAY'S   AND   ZSIGMONDY'S   SOLS. 

derable  admixture  of  the  other  reaction  products, 
and  Carey  Lea  described  it  as  "  allot ropic  silver." 

To  return  to  gold  sols,  a  considerable  step  in 
advance  was  made  by  Faraday  (1857),  who  obtained 
violet  and  purple  liquids  by  reducing  very  dilute 
solutions  of  gold  chloride  with  ethereal  solutions  of 
phosphorus.  He  definitely  expressed  the  view  that 
they  owed  their  colour  to  metallic  gold  in  a  state  of 
fine  division,  noted  the  effect  of  small  quantities  of 
electrolytes  and  even  the  influence  of  the  glass 
vessels  on  the  stability  of  his  preparations.  Some 
of  them  are  preserved  at  the  Royal  Institution,  and 
one  or  two  specimens  still  retain  a  faint  tinge  of 
colour,  while  the  rest  have  coagulated.  A  further 
observation  made  by  Faraday,  that  "  a  little  jelly  " 
increased  the  stability  of  his  preparations,  has  also 
been  confirmed  by  subsequent  investigations  and  has 
acquired  considerable  importance. 

The  modern  history  of  gold  sols  may  be  said  to 
begin  with  Zsigmondy's  paper  on  "  Aqueous  Solu- 
tions of  Metallic  Gold,"  published  in  1898.  This 
describes  the  method,  now  classical,  of  reducing 
alkaline  gold  chloride  solution  with  formaldehyde 
and  is  the  first  research  directed  towards  the  produc- 
tion of  definite  and  reproducible  gold  sols  of  bright 
red  colour.  The  question  of  the  state  of  the  metal 
in  these  solutions  was  definitely  settled  by  their 
examination  in  the  ultra-microscope,  invented  'by 
Zsigmondy  and  Siedentopf  (1903). 

The  last  decade  of  the  nineteenth  and  the  beginning 
of  the  present  century  saw  a  considerable  amount  of 
fundamental  work  on  aspects  of  the  subject  other 
than  the  preparation  of  sols.  Among  the  most 
important  must  be  mentioned  the  investigations  on 
the  effects  of  electrolytes  by  Linder  and  Picton  (1892) 
and  Hardy  (1900).  Research  was  greatly  stimulated 
by  the  invention  of  the  ultra-microscope,  and  in  1909 
and  1910  the  first  works  dealing  comprehensively 


THE   COLLOIDAL  STATE.  5 

and  systematically  with  the  great  mass  of  unco- 
ordinated material  which  had  accumulated  by  then 
were  published  by  R.  Zsigmondy,  Wolfgang  Ostwald 
and  H.  Freundlich. 

The  principal  advance  made  since  Graham's 
fundamental  work  has  been  the  proof,  now  quite 
conclusive,  that  there  are  no  "  Colloids  "  in  Graham's 
sense,  i.e.,  no  definite  class  of  substances  endowed 
with  the  peculiar  properties  designated  by  that  term, 
but  that  by  suitable  methods  any  substance  can,  in  a 
suitable  medium,  be  prepared  in  a  colloidal  condition,- 
which  thus  presents  itself  as  a  state,  not  as  a  form 
of  matter.  Thus  sodium  chloride  is  certainly  a  very 
well-defined  crystalline  substance,  yet  colloidal 
solutions  of  it  in  organic  solvents  can  be  obtained  by 
several  methods. 

P.  P.  von  Weimarn  was  the  first  to  formulate  the 
conditions  necessary  for  obtaining  a  given  body  in 
colloidal  solution  and  succeeded  in  preparing  sols  of 
several  hundred  substances.  Most  of  the  metals  and 
many  non-metallic  elements  have  been  obtained  in 
the  colloidal  state,  and  sols  of  silver,  mercury, 
sulphur,  selenium,  etc.,  are  made  commercially  and 
are  used  in  medicine.  Even  the  alkali  metals  have 
been  obtained  in  the  colloidal  state  by  Svedberg,  by 
the  use  of  organic  solvents,  very  low  temperature  and 
experimental  arrangements  ol  great  ingenuity. 
Similarly,  a  large  number  of  hydroxide  and  sulphide 
sols  are  known,  some  of  which  have  been  the  subjects 
of  classical  investigations,  to  which  we  shall  have 
occasion  to  refer  extensively. . 

All  these  sols  of  inorganic  bodies  are  laboratory 
products  prepared  by  certain  well-defined  methods, 
the  principles  of  which  will  become  clear  as  we  study 
their  properties.  Like  many  laboratory  products  in 
the  domain  of  both  inorganic  and  organic  chemistry 
they  have  been  of  the  greatest  value  in  enabling  us 
to  form  theoretical  conceptions.  Apart  from  the 


6  ORGANIC  COLLOIDS. 

fundamental  characteristic  of  not  diffusing  through 
membranes  like  parchment,  they  have  another 
property  in  common  :  on  the  addition  of  varying, 
but  generally  small  quantities  of  electrolytes,  they 
undergo  marked  and  largely  irreversible  changes,  the 
dissolved  substance  being  precipitated  or  the  whole 
liquid  setting  to  a  jelly. 

In  striking  contrast  to  these  artificially  prepared 
products  there  exists  a  large  group  of  substances, 
which  can  be  dissolved  at  once "  without  special 
methods  to  form  colloidal  solutions  and  are  not, 
indeed,  known  to  form  any  others  or  to  occur  in  the 
crystalline  state.  This  comprises  the  materials  from 
which  all  living  organisms  are  built  up,  like  the 
proteins,  cellulose,  starch  and  many  other  carbo- 
hydrates ;  also  the  various  esters  of  cellulose,  india- 
rubber,  many  dyes,  etc.  Some  of  these,  like  several 
proteins,  dissolve  in  water  alone  ;  cellulose  is  dis- 
solved by  aqueous  solutions  of  copper  oxide- 
ammonia,  zinc  chloride  or  calcium  thiocyanate ; 
while  the  cellulose  esters  and  indiarubber  require 
suitable  organic  solvents.  Even  the  substances 
which  form  sols  with  water  differ  considerably  in 
their  individual  behaviour.  Some,  like  gelatin,  form 
sols  only  above  a  certain  temperature  ;  when  this 
falls  below  a  limit  depending  on  various  factors,  the 
sol  sets  to  a  gel,  the  process  being  reversible,  i.e.,  the 
gel  "  melts  "  again  to  a  sol  on  warming.  On  the 
other  hand,  egg  albumin  is  soluble  at  ordinary 
temperatures,  but  on  heating  above  62°  C.  coagulates 
to  an  insoluble  mass.  Gum-arabic,  to  mention  a 
third  substance  belonging  to  this  class,  forms  a 
viscous  sol  which  neither  sets  to  a  gel  on  cooling 
nor  coagulates  on  heating.  Similarly  varied  is  the 
behaviour  of  these  organic  colloids  towards  electro- 
lytes, but  they  all  share  with  one  another  and  with 
the  inorganic  sols  the  fundamental  characteristic  of  not 
diffusing  through  parchment  or  similar  membranes. 


THE  DISPERSE  SYSTEM.  7 

As  regards  the  sols  in  organic  solvents,  it  must  be 
mentioned  that  the  criterion  of  dialysis  has  hardly 
been  applied  to  them  except  in  isolated  cases.  At 
the  same  time  we  have,  in  the  instances  quoted, 
sufficient  evidence  of  other  kinds  to  the  colloidal 
condition  of  those  substances.  One  of  these  character- 
istics is  an  extremely  low  osmotic  pressure  ;  in  many 
cases  this  is  not  measurable  either  directly  or  by  the 
usual  indirect  methods  (lowering  of  vapour  pressure  or 
of  freezing  point) ,  while  in  others  very  small  pressures 
are  observed,  which  are  strikingly  affected  by  the 
method  of  preparation  arid  by  small  admixtures. 

While  the  recognition  of  the  general  possibility  of 
the  colloidal  state — into  which  all  substances  that 
do  not  assume  it  spontaneously  can  be  brought  TDV^ 
suitable  and  well-defined  methods — constitutes  the 
first  great  advance  since  Graham's  time,  a  step  of 
equal  importance  is  the  demonstration  that  the 
colloidal  state  is  only  a  special  case  of  the  "  disperse 
system,"  a  concept  and  term  first  introduced  by 
Wolfgang  Ostwald.  A  disperse  system  is  a  system 
of  two  phases  differing  in  one  or  more  physical  pro- 
perties, and  having  a  large  surface  of  contact,  or 
interface.  To  obtain  a  surface  large  in  proportion 
to  the  volume  of  at  least  one  phase,  it  is  necessary 
to  reduce  one  or  more  linear  dimensions  of  the  latter  : 
if  one  is  so  reduced,  we  have  films  ;  if  two,  filaments  ; 
and  if  all  three  are  small,  particles  bounded  by  a 
closed  surface,  e.g.,  spherical,  of  one  phase  distributed 
in  the  other.  All  three  configurations  occur  in 
nature  in  many  variations ;  the  type  of  most 
immediate  interest  to  us  is  the  third.  In  such  a 
system  we  can  obviously  pass  in  the  one  phase  from 
one  point  to  any  other,  without  encountering  the 
particles  of  the  other  phase  ;  the  former  is  therefore 
called  the  continuous  (or  "  closed ")  phase,  or  fre- 
quently the  dispersion  medium,  while  the  particles 
constitute  the  disperse  phase. 


8  POSSIBLE  DISPERSE   SYSTEMS. 

While  a  difference  in  one  or  more  physical  pro- 
perties of  the  two  phases,  i.e.,  heterogeneity,  is  part 
of  the  definition,  it  must  not  be  overlooked  that  the 
phases  may,  nevertheless,  have  one  such  property  in 
common.  For  example,  a  suspension  of  glass  in 
cedar  oil  of  the  same  refractive  index  is  optically 
homogeneous,  although  heterogeneous  in  all  other 
respects ;  while  a  suspension  of  particles  in  a  liquid 
of  the  same  density  will  be  homogeneous  as  far  as  the 
effects  of  gravity  or  centrifugal  force  go,  while  its 
optical  heterogeneity  may  be  obvious  even  without 
the  use  of  instruments.  A  single  criterion  is  there- 
fore never  sufficient  to  settle  this  question  definitely, 
a  point  which  is  sometimes  overlooked,  as  we  shall 
have  occasion  to  remark  later  on. 

Differences  in  any  one  physical  property  could  of 
course  form  the  basis  of  a  classification  of  disperse 
systems,  but  the  one  which  has  found  most  general 
acceptance  is  that  introduced  by  Wolfgang  Ostwald, 
which  differentiates  the  disperse  systems  according 
to  the  state  of  aggregation  of  the  phases,  i.e.,  whether 
solid,  liquid  or  gaseous.  As  both  phases  may  be  in 
the  same  state,  nine  cases  altogether  are  possible, 
of  which  one,  however,  is  strictly  limited — that  in 
which  both  phases  are  gaseous.  Since  gases  are 
miscible  in  all  proportions,  we  cannot  have  "  par- 
ticles "  of  disperse  phase  larger  than  molecules. 
The  other  eight  types  are  set  out  below  ;  the  last 
column  gives  examples  of  such  systems  in  the  terms 
in  which  the  principal  representatives  are  generally 
described. 

Dispersion  Disperse 

medium.  phase. 

1.  Gas  . .  Liquid  . .  Fog,  mist. 

2.  Gas  . .  Solid  . .  Smoke,  fine  dust. 

3.  Liquid  .  .  Gas  . .  Froth,  foam. 

4.  Liquid  . .  Liquid  . .  Emulsions. 

5.  Liquid  . .  Solid  . .  Suspensions. 


THE  SPECIFIC  SURFACE.  9 

Dispersion  Disperse 

medium.  phase. 

6.  Solid          . .     Gas         . .      "  Solid  "  froths,  e.g., 

pumice  stone,  etc. 

7.  Solid          . .     Liquid    . .     Numerous  minerals. 

8.  Solid         . .     Solid       . .     Ruby  glass,   many 

coloured  minerals. 

Of  the  above,  4  and  5  will  prove  to  be  of  the 
most  immediate  interest  to  us.  Investigation  has 
shown  that  these  systems  exhibit  the  striking 
features  of  the  colloidal  state  when  the  size  of  the 
particles  falls  between  certain  well-defined  limits, 
and  that  this  is  principally  due  to  the  increase  of 
interface  consequent  on  the  reduction  of  linear 
dimensions.  A  simple  numerical  example  will 
illustrate  this  increase.  M  we  imagine  a  volume  of 
i  c.c.  of  any  material  contained  in  a  cube  of  i  cm. 
edge,  the  total  surface  will  be  6  sq.  cm.  If  we 
reduce  the  linear  dimension  to  one-tenth,  i.e.,  if  we 
divide  the  cubic  centimetre  of  material  into  cubes 
having  an  edge  of  i  mm.  we  obtain  1,000  cubes,  each 
having  a  surface  of  6  sq.  mm.,  so  that  the  total 
surface  is  now  6,000  sq.  mm.  =  60  sq.  cm.,  or  ten 
times  the  surface  of  the  original  cube  with  i  cm.  edge. 
The  surface  therefore  increases  in  the  same,  ratio  in 
which  the  linear  dimension  is  reduced.  If  we 
subdivide  the  cubic  centimetre  into  cubes  with  an 
edge  of  i  [L  (this  is  the  usual  unit  for  microscopic 
measurements)  =  o-ooi  mm.  =  i  x  io~4  cm.,  the 
total  surface  will  be  6  X  io4  sq.  cm.  =  6  square 
metres.  It  will  thus  be  realized  that  still  further 
subdivision  leads  to  such  an  increase  in  surface  per 
unit  mass  that  certain  energies,* of  which  surfaces  are 
the  seat,  may  affect  the  character  of  the  disperse 
system  to  an  extent  equal  with,  or  even  greater 
than,  that  of  the  specific  properties  of  the  disperse 
phase  in  mass.  The  surface  per  unit  volume  of 


lo  DIALYSIS. 

disperse  phase  has  been  called  by  Ostwald  the  specific 
surface. 

The  concept  of  the  disperse  system  is  an  important 
generalization  and  enables  us,  inter  alia,  to  trace  a 
more  or  less  continuous  transition  from  colloidal 
solutions  to  coarser  systems  in  the  one  direction  and 
to  "  molecular  "  and  "  ionic  "  disperse  systems,  i.e., 
true  solutions,  in  the  other.  At  the  same  time,  its 
bearing  on  the  subject  will  be  more  easily  grasped  if, 
instead  of  attempting  to  apply  it  a  priori,  we  proceed 
to  study  the  methods  by  which  colloidal  solutions 
have  been  examined  and  to  learn  how  the  results 
thus  obtained  have  led  to  definite  conclusions 
regarding  the  size  and  state  of  the  disperse  phase. 

Before  entering  upon  this  closer  stud}/,  it  may  be 
well  to  answer  two  questions,  which  will  naturally 
suggest  themselves  to  the  reader  who  has  had  no 
practical  experience  of  the  subject  :  how  is  it 
possible  to  tell  whether  a  given  solution  contains 
colloids  ?  and  how  can  a  few  typical  sols  be  easily 
prepared  ? 

As  regards  the  first  question,  the  fundamental 
method  is  still  dialysis.  Full  details  for  carrying  out 
this,  as  well  as  for  preparing  a  number  of  standard 
sols,  are  given  in  the  author's  "  Laboratory  Manual  " 
(ist  edition,  1920),  to  which  the  reader  is  referred  for 
experimental  procedure  and  technique  generally. 
For  those  unable  to  devote  much  time  to  experi- 
mental work,  the  following  directions  will  be 
sufficient  :— 

The  simplest  contrivance  for  dialysing  aqueous 
solutions  is  a  bag  of  parchment  paper.  A  hexagonal 
or  circular  sheet,  previously  well  soaked  in  distilled 
water,  is  folded  over  the  bottom  of  a  beaker  and  a 
string  tied  round  it  loosely.  The  bag  is  then  slipped 
off  the  beaker,  and  a  string  threaded  through  the 
folds  near  the  mouth,  by  which  it  is  suspended  over 
some  convenient  vessel  so  that  the  bottom  of  the  bag 


TYPICAL   SOLS.  u 

,* 

comes  well  within  the  latter.  The  outside  vessel  is 
now  filled  with  water  up  to  the  bottom  of  the  bag, 
the  solution  to  be  dialysed  poured  slowly  into  the 
latter  and  water  added  outside,  until  the  levels 
inside  and  outside  the  dialyser  are  approximately 
the  same.  The  outside  water  is  changed  at  intervals 
until  it  remains  pure,  which  is  ascertained  by  the 
colour  or  by  tests  with  suitable  reagents.  Whatever 
is  then  left  inside  the  dialyser  is  in  colloidal  solution. 

The  following  sols,  which  are  extremely  easy  to 
prepare,  may  serve  as  representatives  of  important 
types  : — 

Copper  ferrocyanide  sol. — Prepare  solutions  of 
copper  sulphate  and  of  potassium  ferrocyanide,  each 
containing  0-75  gm.  per  litre  of  the  crystallized  salts. 
Pour  a  measured  volume  of  the  former  into  an  equal 
volume  of  the  la.tter  with  constant  stirring.  A  clear 
brown  liquid  results,  which  will  keep  for  a  long  time 
in  stoppered  flasks  of  good  glass. 

Albumin  sol. — Dissolve  5  gm.  of  dried  egg  albumin 
in  100  c.c.  of  0-7  per  cent,  solution  of  sodium  chloride. 
Allow  to  stand  over  night  and  then  filter  through  a 
soft  filter  paper.  If  the  sol  is  to  be  kept,  it  must 
receive  a  small  addition  of  thymol,  to  prevent 
putrefaction. 

Gelatin  sol  and  gel. — Soak  10  gm.  of  leaf  gelatin, 
broken  into  small  pieces,  in  100  gm.  of  water  over- 
night. Warm  on  the  water  bath  until  the  whole 
has  dissolved.  Above  about  27°  C.  the  system  forms 
the  sol ;  below  that  temperature  it  sets  to  a  reversible 
gel,  i.e.,  on  warming  it  is  again  transformed  into  sol. 
This  preparation  must  also  be  preserved  with  a  little 
thymol. 

By  dialysing  the  ferrocyanide  or  the  albumin  sol 
it  can  at  once  be  shown  that  they  conform  to  our 
first  test,  i.e.,  that  they  do  not  diffuse  through  the 
parchment  membrane.  In  the  former  case  the 
outside  water  remains  colourless  ;  in  the  latter  it 


12  EFFECT  OF  ELECTROLYTES. 

can  be  shown  by  any  of  the  well-known  protein 
reactions  to  be  free  from  albumin.  The  effect  of 
electrolytes  on  the  ferrocyanide  sol  may  also  be 
easily  demonstrated  by  adding  to  10  c.c.  of  it  a  few 
drops  of  almost  any  salt  solution  to  be  found  on  the 
laboratory  shelves  ;  the  sol  immediately  becomes 
turbid,  and  after  a  few  hours  the  ferrocyanide 
deposits  as  a  flocculent  precipitate. 


CHAPTER   II. 

IN  the  preceding  chapter  some  general  charac- 
teristics of  colloidal  solutions,  more  particularly  their 
failure  to  diffuse  through  membranes,  have  been 
briefly  referred  to.  It  now  remains  to  describe  in 
some  detail  the  methods  applied  to  the  investigation 
of  colloids  in  more  recent  times,  and  to  see  what 
conclusions  as  to  the  nature  of  the  colloidal  state, 
or,  in  other  words,  the  difference  between  true  and 
colloidal  solutions,  can  be  drawn  from  the  results 
obtained  by  these  various  methods. 

As  regards  dialysis,  we  find  that  parchment  paper 
and  certain  other  septa,  like  collodion  films,  many 
animal  membranes,  etc.,  allow  the  passage  of  true 
solutes,  i.e.,  of  substances  present  in  the  solvent  as 
molecules  or  as  ions,  while  they  retain  colloids.  If 
we  ask  ourselves  for  the  reason  of  this  phenomenon, 
the  simplest  answer — though  by  no  means  a  complete 
one — is  obviously  that  the  colloids  are  present  as 
particles  or  aggregates  too  large  to  pass  through  the 
pores  in  the  membrane.  They  may,  of  course, 
actually  have  molecules  of  such  excessive  size — thus 
Congo  Red,  a  dye  which  lies  on  the  border  between 
true  and  colloidal  solutions,  has  a  molecule  consisting 
of  72  atoms  with  a  molecular  weight  of  654 — and 
bodies  which  also  form  spontaneously  colloidal 
solutions,  like  albumin  and  other  proteins,  certainly 
have  molecules  of  still  more  considerable  size.  On 
the  other  hand,  this  explanation  seems  hardly 
applicable  to  the  inorganic  and  particularly  the 
metal  sols,  and  it  is  necessary  to  assume  that  these 
contain  aggregates  formed  of  a  large  number  of 


14  SIZE  OF  PARTICLES. 

molecules — an  assumption  for  which,  as  we  shall  see 
later  on,  there  is  now  direct  experimental  evidence. 

It  is,  however,  impossible  to  look  on  dialysis  as  a 
simple  "  sieve  "  action  of  the  septum,  because 
certain  little  understood  relations  between  the 
solvent  and  the  membrane  are  necessary  to  permit 
the  latter  to  pass  even  solvent  alone.  Even  if  it 
were  possible,  the  fact  that  dialysis  is  carried  on 
without  any  pressure  would  make  conclusions 
uncertain  :  it  is  quite  usual  to  retain  by  ordinary 
filter  media  under  low  pressure  particles  that  are 
much  smaller  than  the  pores  of  the  former  ;  thus 
sand  niters  retain  bacteria,  although  they  are  very 
much  smaller  than  the  interstices  between  the  sand 
grains.  To  eliminate  this  difficulty,  and  also  with 
the  purpose  of  separating  the  disperse  phase  from  fhe 
dispersion  medium,  and  not  only  from  the  true 
solutes  which  may  be  present,  various  investigators 
have  attempted  to  retain  colloids  by  nitration  under 
pressure  through  very  dense  media,  such  as  the  filter 
candles  used  in  bacteriological  work.  Linder  and 
Picton  used  the  latter  in  the  course  of  their  classical 
investigations  on  arsenic  trisulphide  sols,  which  were 
found  to  vary  according  to  the  method  of  prepara- 
tion. Most  of  them  passed  through  the  filter 
unaltered,  but  a  portion  of  the  sulphide  from  certain 
sols  was  retained.  For  these  isolated  cases  a  limit 
value  for  the  size  of  the  particles  could  -thus  be 
deduced. 

A  step  in  advance  was  taken  by  C.  J.  Martin,  who 
used  as  filtering  media  gels,  e.g.,  of  silicic  acid,  and 
employed  very  considerable  pressure.  He  found  it 
possible  by  this  procedure  to  retain  proteins  from 
their  sols,  but  left  the  question  of  the  size  of  particles  „ 
open.  Filtration  through  gels  was  eventually  deve- 
loped by  H.  Bechhold  into  a  fairly  simple  procedure, 
with  only  moderate  pressures — rarely  more  than  five 
atmospheres — and  called  by  him,  in  allusion  to  the 


ULTRA-FILTRATION.  15 

ultra-microscope  "  Ultra- Filtration."  Membranes 
similar  in  nature  to  those  used  in  dialysis  are 
employed,  but  the  outside  is  not  submerged  in  the 
solvent.  To  permit  the  use  of  pressure,  the  mem- 
branes are  prepared  as  follows  :  strong,  hard  filter 
paper  is  impregnated,  preferably  in  vacua,  with 
either  a  gelatin  sol  or  acetic  acid  collodion  of  known 
concentration.  The  gelatin  filters  are  then  immersed 
in  cold  formaldehyde  for  several  days  and  rendered 
insoluble,  while  the  collodion  filters  are  immersed  in 
water,  which  gradually  replaces  the  acetic  acid  and 
leaves  a  gelatinous  mass  of  nitro-cellulose  in  the 
substance  of  the  paper.  The  filters  are  clamped  in  a 
small  pressure  vessel  and  are  supported  on  wire 
gauze  and  perforated  metal,  so  that  pressures  up  to 
10  atmospheres  can  be  used,  if  necessary.  A  very 
important  feature,  predicted  and  subsequently 
verified  by  Bechhold,  is  the  ease  with  which  the 
porosity  of  these  filters  can  be  varied  by  altering  the 
concentration  of  the  original  gelatin  or  collodion,  so 
that  particles  which  pass  freely  through  a  "  2-5  per 
cent."  collodion  filter,  i.e.,  one  made  from  a  collodion 
containing  2-5  gm.  of  nitro-cellulose  in  100  c.c.,  can 
be  completely  retained  by  one  made  from  5  per  cent, 
collodion. 

The  size  of  the  pores  in  these  septa,  which  interests 
us  at  the  moment,  can  be  determined  by  two  methods 
indicated*  by  Bechhold  ;  either  by  ascertaining  the 
pressure  required  to  force  air  through  a  membrane 
saturated  with  water,  or  by  measuring  the  volume  of 
water  forced  through  unit  area  in  unit  time  by  a 
known  pressure.  Both  calculations  involve  a  con- 
siderable number  of  simplifying  assumptions,  and 
a  very  high  degree  of  accuracy  cannot  be  expected 
from  them,  though  there  is  no  doubt  about,  the 
results  being  of  the  correct  order.  A  third  method 
was  indicated  by  the  author,  and  consists  in  deter- 
mining the  pressure  at  which  oil  globules  of  known 


16  THE  TYNDALL  CONE. 

size,  suspended  in  water,  are  just  forced  through  the 
membrane.  If  the  interfacial  tension  oil- water  is 
known,  the  radius  of  the  pores — assumed  to  be  of 
circular  section — can  be  calculated  with  a  fairly  high 
degree  of  accuracy. 

The  result  of  Bechhold's  determinations  is  that, 
according  to  the  concentration  of  the  collodion  or 
gelatin  used,  the  diameters  of  the  pores  lie  between 
930  jitju,  and  21  jLt/x  (the  jitju,,  which  is  the  unit  generally 
employed  in  giving  the  dimensions  of  such  particles 
as  we  shall  have  to  deal  with,  is  o-ooi  /x,  and  there- 
fore one-millionth  millimetre,  i.e.,  i  X  io~6  mm. 
=  i  X  io~7  cm.).  These  dimensions  give  us  limits 
for  the  sizes  of  the  particles  retained  by  such  filters ; 
if  the  particles  are  retained,  they  are  probably,  though 
not  necessarily,  larger  than  the  pores  ;  if  they  pass 
through  the  filter,  it  is  reasonably  certain  that  they 
are  much  smaller  than  the  pores. 

Evidence  tending  in  the  same  direction  and  towards 
the  same  limits  is  afforded  by  an  optical  property  of 
many  sols.  Although  the  latter  may  be  perfectly 
clear  in  transmitted  light,  the  path  of  an  intense  beam 
of  light  projected  through  them  and  viewed,  best 
against  a  dark  background,  at  right  angles  to  its 
direction,  becomes  clearly  visible,  the  liquid  appearing 
either  more  or  less  turbid,  while  sometimes  exhibiting 
a  different  colour  from  that  shown  in  transmitted 
light.  The  phenomenon  was  noticed  already  by 
Faraday  in  his  gold  sols ;  it  was  subsequently 
employed  by  Tyndall  in  the  study  of  fogs,  etc.,  and 
is  generally  called  the  Tyndall  cone.  If  the  cone  is 
viewed  through  some  suitable  analyzer,  it  is  found  to 
be  polarized,  and  it  must  be  emphasized  that  herein 
lies  its  difference  from  true  fluorescence  ;  the  blue 
cone. seen  under  similar  conditions  of  illumination  in 
a  solution  of  quinine  sulphate,  or  in  certain  petro- 
leums, is  not  polarized.  The  whole  phenomenon  has 
been  investigated  mathematically  by  Lord  Rayleigh, 


SLIT  ULTRA-MICROSCOPE.  17 

who'  proved  that  to  produce  it,  the  particles  in  the 
path  of  the  beam  must  be  small  compared  with  the 
wave-length  of  light.  The  values  of  the  latter  are,  of 
course,  known  with  very  great  accuracy,  and  lie 
between  450  and  760  ^  for  the  visible  spectrum. 
These  limits,  again,  agree  well  with  those  deduced 
from  the  study  of  ultra-filters. 

At  the  same  time,  they  answer  a  question  which 
may  already  have  occurred  to  the  reader  :  why  the 
size,  or  at  least  the  presence  of  such  particles  as  may 
be  present  in  a  sol,  cannot  be  ascertained  directly 
by  microscopic  observation  ?  The  answer  is,  that 
objects  small  in  comparison  with  the  wave-length  of 
light  are  invisible  in  the  ordinary  microscope.  The 
limit  of  resolving  power  of  the  best  microscope  is 
0-2  JJL  -=  200  /x/x,  which  does  not,  however,  mean  that 
particles  of  this,  or  larger  size,  can  necessarily  be 
made  visible. 

It  has,  however,  been  known  for  some  time  that 
with  favourably  arranged  illumination  objects  even 
of  submicroscopic  dimensions  could  be  rendered 
visible.  Thus  it  had  been  shown  by  Fizeau  and  by 
Ambronn  that  slits  of  much  smaller  width  than  the 
limit  of  resolution  could  be  seen  if  strongly  illumi- 
nated on  a  dark  ground.  These  observations  sug- 
gested to  Zsigmondy  and  Siedentopf  the  possibility 
of  rendering  visible  the  individual  particles  which 
collectively  produce  the  Tyndall  phenomenon,  if  only 
the  light  scattered  by  the  particles  (reflection  in  the 
ordinary  sense  cannot  take  place  from  objects 
smaller  than  the  wave-length  of  light)  was  permitted 
to  enter  the  microscope,  but  no  direct  rays  from  the 
source  of  light. 

This  expectation  has  been  fully  realized  and  the 
"  ultra-microscope  "  has  now  become  a  familiar  and 
indispensable  instrument  of  research.  The  name 
may  be  misleading,  unless  it  is  remembered  that  the 
microscope  used  is  of  the  ordinary  kind,  arid  that 


i8  SLIT  ULTRA-MICROSCOPE. 

only  the  method  of  illuminating  the  object  is  different. 
The  principle  of  the  latter  is  very  simple  ;  a  powerful 
beam  of  light  is  thrown  horizontally  through  a  small 
body  of  liquid,  and  the  illuminated  volume  is 
observed  through  a  microscope,  the  -axis  of  which  is 
vertical.  It  is  at  once  obvious  that  no  light  enters 
the  instrument  except  such  as  has  been  scattered  by 
particles  present  in,  and  optically  different  from,  the 
liquid  itself,  i.e.,  they  must  be  opaque  or  possess  a 
refractive  index  different  from  that  of  the  latter. 

Any  particles  present  under  these  conditions 
appear  as  light  discs  on  a  dark  background.  The 
images  are  not  geometrical  images,  and  their  apparent 
size  is  not  increased  by  the  use  of  higher  powers.  The 
effect  of  the  latter  is  merely  increased  magnification 
of  the  distances  between  particles  and  of  any  motion 
which  they  may  possess.  The  visibility  depends  on 
the  intrinsic  brilliancy  of  the  source  of  light  and  on 
the  optical  difference  between  the  phases  ;  if  the 
latter  is  great,  particles  as  small  as  5  /x/z  diameter  can 
still  be  distinguished  with  direct  sunlight.  In  many 
sols,  however,  particularly  organic  ones,  no  particles 
can  be  seen  even  in  the  most  favourable  conditions, 
but  only  a  diffuse  light.  This  may  be  due  to  their 
small  size,  but  the  other  possibility,  an  insufficient 
optical  difference  between  disperse  phase  and  disper- 
sion medium,  must  not  be  overlooked.  We  shall 
see  that  in  many  cases  the  disperse  phases  must  be 
considered  to  contain  considerable  amounts  of  solvent 
in  some  way  associated  with  their  substance,  and  the 
optical  difference  may  thus  become  very  small, 
while  the  particles  are  likely  to  be  comparatively 
large. 

Since  the  images,  as  has  already  been  explained, 
are  not  geometrical,  i.e.,  bear  no  known  ratio  to  the 
size  of  the  object,  direct  measurements  are  still 
impossible.  The  size  of  particles  seen  in  the  ultra- 
microscope  can  however  be  calculated  by  an  indirect 


SLIT  ULTRA-MICROSCOPE.  19 

method,  which  will  be  more  easily  understood  after 
a  short  description  of  the  instrument  as  actually 
used.  This  is  shown  in  Fig.  i.  The  light  of  a  large 
arc  lamp  d  is  projected  by  the  lens /on  a  "  precision  " 
slit  g,  the  width  and  height  of  which  can  be  adjusted 
-very  accurately,  and  which  can  be  rotated  round  an 
horizontal  axis.  An  image  of  the  slit  is  formed  by 
the  second  lens  h  and  projected  on  the  illuminating 


FIG.    i. — GENERAL   ARRANGEMENT    OF  THE    SLIT    ULTRA- 
MICROSCOPE. 

device  proper  I,  which  is  substantially  a  microscope 
objective.  This  throws  an  intense  beam  of  light 
through  a  cell  containing  the  liquid  to  be  examined, 
which  is  seen  more  distinctly  in  Fig.  2,  where  it  is 
shown  in  position  on  the  microscope.  It  is  of 
rectangular  cross -section  and  has  two  windows — the 
one  in  front  admits  the  beam  of  light,  while  the 
second  is  at  the  top  opposite  the  objective  of  the 
microscope.  The  cell  is  provided  with  a  funnel  at 
one  end  and  an  outlet  at  the  other,  so  that  a  large 
volume  of  liquid  can  be  passed  through  and  examined 
at  one  setting. 

2—2 


20 


SIZE  OF  PARTICLES. 


A  small  portion  of  the  illuminated  volume  can  be 
delimited  and  the  length — in  the  direction  of  the  axis 


X  A  /.f  HUNSERi  JENA. 


FIG.  2. — MICROSCOPE  WITH  QUARTZ  CHAMBER  FOR  ULTRA- 
MICROSCOPIC  EXAMINATION  OF  LIQUIDS. 

of  the  beam — as  well  as  the  width  measured  directly 
by  means  of  an  eye -piece  micrometer  ;  the  depth  is 
determined  by  turning  the  slit  90°  so  that  what  was 
the  depth  of  the  illuminated  prism  can  now  also  be 
measured.  The  sol  is  diluted  so  far  that  only  a 


SIZE  OF  PARTICLES.  21 

small  number  of  particles  are  visible  in  this  known 
volume,  and  their  average  number  is  determined  by 
repeated  counts.  The  weight  of  disperse  phase  in 
unit  volume  is  known  from  its  method  of  preparation, 
and  the  weight  present  in  the  observed  volume,  which 
is,  of  course,  of  the  order  of  cubic  p,,  is  calculated 
therefrom.  Dividing  this  weight  by  the  number  of 
particles  observed,  we  obtain  the  weight  of  one 
particle,  and  we  can  then  calculate  its  dimensions  by 
making  two  assumptions  :  i.  That  the  density  of 
the  particles  is  the  same  as  that  of  the  material  in 
bulk ;  and  2.  That  the  particles  have  a  simple 
geometrical  shape,  e.g.,  spherical  or  cubical.  In  the 
generality  of  cases  probably  neither  assumption  is 
correct,  but  there  is  no  doubt  about  the  order  of 
magnitude  thus  calculated  being  right.  Very  many 
determinations  of  this  kind  have  been  made  by 
numerous  observers,  since  the  invention  of  the 
ultra-microscope,  and  we  shall  have  occasion  to  refer 
to  them  again. 

Particles  visible  in  the  ordinary  microscope  are 
generally  described  as  microns,  those  which  can  be 
made  visible  by  the  ultra-microscope  as  submicrons, 
and  those  which  cannot  be  rendered  visible  even  by 
the  latter,  as  amicrons.  As  has  already  been 
pointed  out,  however,  these  differences  are  not 
determined  by  the  size  alone. 

The  apparatus  described  is  the  most  perfect  one 
for  the  observation  of  ultra-microscopic  particles, 
and  the  only  one  which  can  be  used  for  determining 
their  size.  As  it  is  rather  costly  and  requires 
powerful  illumination,  a  number  of  devices  have 
been  introduced  with  which  the  ordinary  axial 
illumination  can  be  employed,  while  direct  light — 
other  than  that  scattered  by  particles — is  still 
prevented  from  entering  the  microscope.  In  a 
number  of  them  this  object  is  effected  by  total" 
reflection  from  the  cover  glass  ;  the  principle  will  be 


22  DARK   GROUND   CONDENSERS. 

readily  understood  by  reference  to  Fig.  3,  which 
shows  a  section  of  the  "  paraboloid  "  condenser  made 
by  Carl  Zeiss.  The  condenser  is  part  of  a  paraboloid 
of  revolution,  bounded  by  two  parallel  planes  at 
right  angles  to  the  axis.  Parallel  rays  entering  the 
condenser  axially  are,  as  is  well  known,  reflected  into 
the  focus  of  the  paraboloid,  and  the  top  face  of  the 
condenser  is  so  adjusted  that  this  focus  falls  on  the 
surface  of  the  slide,  which  is  of  a  definite  thickness. 

A  central  stop  covers  part 
of   the   bottom   face    and 
\  /  permits  only  such  rays  to 

t ,L*c*«aife .       Pass  as  will,  after  reflec- 
tion,   strike    the    surface 
under    an    angle    greater 
than  the  critical  angle,  so 
that  they  would  be  totally 
reflected  by  the  top  of  the 
condenser    if    in    contact 
with  air.    By  placing  cedar 
oil   between    this    surface 
FIG.   3. — SECTION  OF  PARABO-   and  the   slide,  the  light  is 
LOID  CONDENSER,  SHOWING  enabled  to   pass  through 
PATH  OF  RAYS.  the  latter  and  any 


placed  on  it  for  examina- 
tion, but  is  totally  reflected  at  the  cover  glass  which 
rests  on  this  liquid.  The  field  therefore  is  dark  if 
the  liquid  is  quite  free  from  particles,  while,  if  they 
are  present,  the  light  scattered  from  them — as  indi- 
cated by  the  dotted  lines — can  enter  the  microscope 
and  form  an  image. 

The  use  of  slides  and  cover  glasses  involves  several 
disadvantages  ;  these  are  avoided  by  the  design 
illustrated  in  Fig.  4,  which  represents  a  section 
through  the  Jentzsch  "  ultra-condenser."  The  liquid 
is  placed  directly  in  the  spherical  hollow  a,  which  is 
closed  by  a  quartz  cover  (not  shown),  and  holds 
about  i  c.c.  The  lower  face  of  the  condenser  is 


ULTRA-CONDENSER. 


provided  with  a  central  stop.  The  rays  which  enter 
are  twice  reflected  as  shown  by  the  dotted  lines,  and 
come  to  a  focus  in  the  axis  at  some  point  near  the 
top  of  the  cavity  a.  A  thin  layer  of  the  liquid  is  thus 
illuminated  very  intensely,  while  no  direct  light 
passes  axially.  The  particles  visible  are  observed 
at  some  distance  from  any  glass  surface,  which 
eliminates  various  disturbing  factors.  A  small  hand- 
regulated  arc  must  be  used  as  source  of  light,  while 
a  large  incandescent  gas 
burner,  or  a  "  Point  o' 
light  "  lamp,  is  sufficient 
for  the  slide  and  cover- 
glass  types. 

All  these  appliances  can 
be  fitted  to  ordinary 
microscopes  and  permit  a 
rapid  diagnosis,  though 
only  the  "  ultra-conden- 
ser "  approaches  the  slit 
ultra  -  microscope  when 
very  small  particles  are  FlG-  4-— SECTION  OF  ULTRA- 
ID  be  rendered  visible.  _  £'S5J.ERi  SHOWING  ?ATH 

The  result  of  our  brief 

and  general  survey  of  the  various  methods  for 
examining  colloidal  solutions  is  that  the  majority 
contain  the  disperse  phase  as  particles  below  the 
limit  of  microscopic  visibility,  but  capable  of  being 
rendered  visible  and  measured,  as  well  as  capable  of 
being  retained  by  certain  porous  septa.  The  sizes  at 
which  we  arrive  by  these  methods  are  still,  even  in 
the  cases  of  the  smallest  submicrons,  considerably 
larger  than  the  limit  values  for  the  size  of  various 
molecules  deduced  by  numerous  investigators  em- 
ploying a  great  variety  of  methods. 

In  this  presence  of  particles  of  sizes  greatly 
exceeding  molecular  dimensions  must  be  sought  one 
of  the  fundamental  differences  between  colloidal  and 


24  SIZE  AND  OTHER  FACTORS. 

true  solutions.  While  this  characteristic,  or  rather 
the  order  of  magnitude,  alone  would  explain  certain 
properties  of  sols,  such  as  their  extremely  low  osmotic 
pressure,  it  fails,  by  itself,  to  account  for  others  and 
more  especially  for  those  in  which  various  sols  differ 
very  strikingly  from  one  another.  We  shall  discuss 
these  differences  and  the  way  in  which  they  lead  to 
a  classification  of  sols  in  the  next  chapter. 


CHAPTER   III. 

A  PROPERTY  which  varies  very  markedly  in 
different  sols  is  their  behaviour  to  electrolytes.  The 
metal  and  sulphide  sols,  or  the  copper  ferrocyanide 
sol  described  on  p.  n,  show  an  immediate  change  on 
the  addition  of  electrolytes  even  in  small  concentra- 
tions ;  they  either  change  colour  or  become  turbid, 
and  after  a  certain  time  the  whole  of  the  disperse 
phase  settles  out,  leaving  the  supernatant  dispersion 
medium  clear.  An  addition  of,  say,  i  c.c.  of  normal 
sodium  chloride  solution  to  19  c.c.  of  the  copper 
ferrocyanide  sol  will  produce  these  changes  within  a 
few  minutes.  On  the  other  hand,  the  addition  of 
this  amount  of  sodium  chloride  solution  to  the  albumin 
sol  will  not  produce  any  obvious  change  at  all,  and 
even  very  much  greater  quantities  will  fail  to  do  so. 
Other  sols,  while  still  extremely  sensitive  to  electro- 
lytes, undergo  a  different  kind  of  change  ;  the  dis- 
perse phase  is  not  separated  as  precipitate,  but  the 
whole  liquid  sets  to  a  coherent  gel.  Silicic  acid  sol 
and  eerie  hydroxide  sol  are  examples  of  this  type, 
the  latter  a  particularly  striking  one. 

This  difference  in  the  behaviour  towards  electro- 
lytes has  been  made  the  basis  of  a  classification  by 
Perrin  and  by  Freundlich,  who  divide  colloids  into 
"  lyophobe  "  and  "  lyophile,"  i.e.,  such  as  remain 
reluctantly,  and  such  as  remain  freely  in  solution,  in 
presence  of  electrolytes.  While  this  is  no  doubt  an 
important  criterion,  the  distinction  is  purely  descrip- 
tive, and  we  shall  endeavour  to  find  a  classification 
which  goes  more  directly  to  the  root  of  the  matter, 
viz.,  that  first  proposed  by  Wolfgang  Ostwald. 


26         GROUNDS   OF   CLASSIFICATION. 

This  takes  as  its  basis  the  state  of  aggregation  of  the 
phases,  in  other  words  it  looks  on  colloidal  solutions 
as  special  cases  of  the  disperse  system.  The  disper- 
sion medium  in  the  types  of  immediate  interest  to  us 
is  liquid,  so  that  the  classification  rests  on  the  state 
of  aggregation  of  the  disperse  phase,  which  may  be 
solid,  liquid  or — though  this  hardly  concerns  us — 
gaseous. 

We  could  probably  predict  some  properties  of 
systems  having  particles  within  the  colloidal  range 
of  sizes  and  either  solid  or  liquid,  but  it  is  both  easier 
and  more  convincing  to  proceed  by  way  of  analogy 
from  coarser  systems.  One  property  which  exhibits 
very  marked  differences  in  different  types  of  sols, 
and  does  so  equally  in  coarse  systems  composed  of 
known  phases,  is  the  viscosity,  and  we  will  consider 
these  differences  in  some  detail. 

The  reader  is  no  doubt  aware  of  what  is  under- 
stood, in  a  general  way,  by  the  viscosity  of  a  liquid  : 
the  resistance  offered  to  shearing,  stirring  or  the  flow 
through  a  capillary  tube.  If  a  liquid  is  contained 
between  two  parallel  plates  and  one  of  them  is  moved 
with  constant  velocity  in  its  own  plane,  a  certain 
force  is  required  to  maintain  this  velocity,  which 
depends  on  the  latter,  the  area  and  distance  of  the 
plates  and  on  .the  nature  and  temperature  of  the 
liquid.  This  gives  us  the  definition  of  the  viscosity 
coefficient,  at  any  given  temperature  :  the  force  required 
to  move  a  plate  of  unit  surface  separated  from  a  plate 
of  the  same  size  by  a  layer  of  liquid  of  unit  thickness, 
at  unit  velocity.  Such  coefficients  for  many  liquids, 
expressed  in  absolute  units,  can  be  found  in  the 
various  tables  of  physical  constants.  They  all 
decrease  rapidly  with  rising  temperature,  the  decrease 
in  many  cases  amounting -to  several  per  cent,  per 
degree  C. 

The  viscosity  cannot  be  conveniently  measured 
by  any  method  directly  embodying  the  definition, 


VISCOSITY   OF  SOLS.  27 

but  the  coefficient  can  be  deduced  from  the  time  of 
flow  through  a  capillary  under  known  conditions. 
The  text-books  of  physics  should  be  consulted  for 
the  theory  and  details  of  the  method  and  for  the 
various  criteria  to  be  satisfied  if  the  measurements 
are  to  be  correct. 

As  regards  colloidal  solutions,  they  fall  very  dis- 
tinctly into  two  classes  in  respect  of  the  increase  in 
viscosity  over  that  of  the  pure  dispersion  medium, 
produced  by  a  given  percentage  of  disperse  phase. 
One  class,  the  metal  and  sulphide  sols  in  particular, 
shows  a  viscosity  only  very  slightly  higher  than  that 
of  water.  The  other,  which  comprises  principally 
the  organic  colloids,  such  as  albumin,  gelatin,  agar, 
etc.,  shows  a  very  considerable  increase  of  viscosity, 
even  with  small  percentages  of  dissolved  matter. 
Even  more  striking  is  the  increase  caused  by  certain 
colloids  in  organic  dispersion  media  ;  rubber  sols 
containing  i  per  cent,  of  rubber  in  benzene  may  have 
a  viscosity  60  to  100  times  that  of  pure  benzene, 
while  sols  of  nitro-cellulose  of  the  same  concentration 
may  have  viscosities  several  hundred  times  greater 
than  the  dispersion  medium. 

Coarse  systems  show  very  similar  differences.  It 
is  a  fact  familiar  to  everybody  who  has  stirred  up 
finely  divided  solid  matter,  such  as  precipitates  of 
calcium  carbonate  or  of  barium  sulphate,  with  water, 
that  even  with  20  or  30  per  cent,  the  mixture  does 
not  offer  any  great  resistance  to  stirring,  i.e.,  the 
viscosity  is  not  much  increased.  On  the  other  hand, 
it  is  equally  well  known  that  systems  of  two  liquid 
phases  insoluble  in  each  other,  generally  called 
emulsions,  show  a  viscosity  much  higher  than  that 
of  either  phase,  though  it  is  not  realized  how  great 
this  increase  may  become.  Various  pharmaceutical 
and  domestic  preparations  are  familiar  to  everybody  ; 
a  good  example  of  the  latter  is  mayonnaise  sauce,  an 
emulsion  of  oil  in  yolk  of  egg.  The  viscosity 


28  SOLID  PARTICLES. 

increases  steeply  with  increasing  percentage  of 
disperse  phase,  extreme  cases  being  represented  by 
certain  emulsions  used  as  "  solid  "  lubricants,  and 
by  Pickering's  emulsions  with  99  per  cent,  of  mineral 
oil  in  i  per  cent,  of  soap  solution,  which  could  be  cut 
into  cubes. 

Applying  these  considerations  to  sols,  we  are  led 
to  the  conclusion — now  fairly  generally  accepted — 
that  in  those  which  show  a  low  viscosity  the  disperse 
phase  is  present  as  solid  particles,  while  in  the  sols 
with  high  viscosity  the  disperse  phase  is  liquid.  The 
distinction  between  solid  and  liquid  involves  some 
difficulties  when  applied  to  ultra-microscopic  bodies, 
which  will  be  referred  to  again  ;  as,  however,  the 
particles  must  consist  of  a  number  of  molecules,  it  is 
obviously  possible  that  in  one  case  they  may  be 
held  together  by  the  forces  characteristic  of  the  solid 
state,  while  in  the  other  the  cohesion  may  be  of  the 
same  kind  as  in  liquids,  viz.,  accompanied  by  relative 
mobility,  although  we  shall  find  this  restricted  by  the 
action  of  the  surface.  That  the  former  assumption 
is  correct,  at  least  in  one  typical  case,  has  been  proved 
by  Debye  and  Scherrer  (Phys.  Zeitschr.,  ij,  277, 
1916)  by  X-ray  analysis  of  the  particles  of  protected 
gold  sols.  They  invented  a  method  for  applying  this 
to  arbitrarily  orientated  particles  and  found  the 
space  lattice  of  the  ultra-microscopic  particles  to 
be  exactly  the  same  as  that  of  large  gold  crystals,  a 
result  which  leaves  no  doubt  regarding  the  state  of 
aggregation. 

No  such  direct  demonstration  is  possible  in  the 
other  class  of  sols,  and  the  first  difficulty  is  that  the 
disperse  substance  in  its  original  state,  such  as  dry 
albumin  or  dry  nitro -cellulose,  is  not  liquid.  We 
have,  therefore,  to  make  a  further  assumption  : 
that  the  substance,  in  being  dispersed,  becomes 
associated  with  considerable  amounts  of  the  disper- 
sion medium,  and  that  these  highly  hydrated  (or, 


ANOMALIES  OF  VISCOSITY.  29 

in  media  other  than  water,  "  solvated  ")  aggregates 
constitute  the  disperse  phase  and  possess  the  free 
deformability  of  the  liquid  state.  This  conception 
is  not  an  easy  one,  but  it  rests  fortunately  on  other 
evidence  as  well  as  the  mere  analogy  in  viscosity. 
The  latter  is  not  only  high,  but  exhibits  anomalies 
that  distinguish  it  sharply  from  that  of  homogeneous 
liquids  with  very  high  viscosity,  such  as  glycerin  or 
castor  oil.  The  viscosity  of  most  sols  of  this  class 
is  not  a  real  constant  for  a  given  temperature  ;  it 
may  decrease  (eerie  hydroxide  sol)  or  increase 
(gelatine,  silicic  acid  sol)  merely  with  age.  By 
appropriate  methods  it  can  further  be  shown,  that 
at  any  given  time  the  viscosity  is  not,  as  with 
homogeneous  liquids,  independent  of  the  rate  at 
which  the  liquid  is  sheared,  but  varies  enormously 
with  this  rate.  This  anomaly  was  first  investigated 
by  H.  Garrett  (Dissertation,  Heidelberg,  1903),  and 
many  measurements  revealing  it  have  been  carried 
out  by  the  author  (Koll.-Zeitschr.,  jj,  88,  1913). 
Even  somewhat  prolonged  shearing  at  the  same  rate 
may  alter  the  viscosity  markedly.  A  further  factor 
to  be  considered  is  the  decrease  in  viscosity  with 
rising  temperature.  As  we  have  already  mentioned, 
all  liquids  show  this  decrease,  but  if  we  compare  the 
temperature  coefficient  of  the  viscosity  of,  say,  a 
gelatine  sol  with  that  of  the  dispersion  medium, 
water,  we  find  the  former  enormously  greater.  This 
fact  is  probably  most  easily  and  naturally  explained  by 
a  change  in  the  degree  of  hydration,  which  involves 
a  transfer  of  water  from  one  phase  to  the  other. 

Further  and  more  direct  evidence  of  the  state  of 
aggregation  of  the  disperse  phase  is  furnished  by 
the  fact  that  some  sols  of  the  class  we  are  describing, 
such  as  gelatin  or  soap  sols,  can,  by  the  addition  of 
neutral  salts,  be  separated  into  two  liquid  layers, 
both  of  which  contain  the  dispersed  substance,  but 
in  very  different  concentrations.  An  experiment  by 


30  OSTWALD'S   CLASSIFICATION. 

W.  Pauli  and  P.  Rona  shows  this  very  convincingly  : 
a  10  per  cent,  gelatin  sol  receives  an  addition  of 
sodium  sulphate  at  30°  C.,  which  produces  a  copious 
coagulum.  On  allowing  the  preparation  to  stand 
for  several  hours  at  the  same  temperature,  it  will  be 
found  that  the  coagulum  has  completely  coalesced 
to  a  liquid  layer  of  concentrated  gelatin,  which  is 
separated  by  a  sharp  boundary  from  a  supernatant 
layer  of  dilute  gelatin  sol.  Since  it  is  certain — from 
the  analogous  effect  of  salts  on,  say,  a  mixture  of 
alcohol  and  water — that  the  first  effect  of  the  salt  is 
a  withdrawal  of  water,  and  since  the  portion  of  the 
system  from  which  the  water  has  been  removed 
proves  still  to  be  liquid,  it  is  only  reasonable  to  infer 
that  it  was  liquid  when  it  contained  more  water,  viz., 
in  its  initial  condition. 

Disperse  systems  with  liquid  dispersion  medium 
and  solid  disperse  phase  are  known  as  suspensions, 
while  those  with  liquid  dispersion  medium  and 
liquid  disperse  phase  are  called  emulsions  (see  p.  8). 

Wolfgang  Ostwald  accordingly  calls  the  sols  with 
solid  disperse  phase  "  Suspensoids,"  and  those  with 
liquid  disperse  phase  "  Emulsoids,"  which  ter- 
minology will  be  used  throughout  this  book.  The 
two  classes  coincide  to  some  extent,  though  by  no 
means  completely,  with  the  "  lyophobe "  and 
"  lyophile  "  colloids  respectively. 

The  suspensoids  show  a  much  more  uniform 
behaviour  towards  various  influences  than  the 
emulsoids,  and  will,  therefore,  be  considered  first. 
We  are  now  in  the  possession  of  two  data  :  we  know 
the  approximate  size  of  the  particles,  and  we  have 
concluded  that  they  are  solid.  The  next  step  is  to 
co-ordinate  these  two  factors  by  examining  the 
behaviour  of  small  particles  suspended  in  a  liquid, 
and  especially  to  study  how  this  behaviour  changes 
when  their  size  decreases  to  ultra-microscopic  dimen- 
sions. 


CHAPTER   IV. 

IT  was  shown  by  Stokes  in  1850  that  a  small 
sphere  falling  in  a  liquid  soon  assumes  a  constant 
velocity,  which  is  given  by  a  formula  that  has  since 
played  a  part  in  an  enormous  number  of-  most 
important  investigations.  The  formula  applies 
strictly  only  when  the  liquid  is  infinitely  extended 
and  corrections  have  to  be  made  if  the  sphere  falls 
in  the  vicinity  of  a  wall  or  in  a  narrow  vessel.  If  we 
call  :— 

r  the  radius  of  the  particle, 
s  the  specific  gravity  of  the  same, 
s'  the  specific  gravity  of  the  liquid, 
17  the  viscosity  of  the  latter, 
g  the  gravity  constant, 
the  constant  velocity  of  the  particle  is  :— 

v  =  2r2(s  -  s')g 

9r, 

It  is  obvious  that  the  difference  (s  —  s')  may  be 
positive,  zero  or  negative — that  is,  the  particle  may 
sink,  remain  stationary  or  rise,  if  its  specific  gravity 
is  greater,  equal  to,  or  smaller  than  that  of  the 
liquid.  It  is  also  obvious  that,  other  things  being 
equal,  the  velocity,  in  whichever  direction,  is 
inversely  proportional  to  the  viscosity  of  the  liquid  : 
a  particle  of  given  size  and  weight  will  sink  several 
hundred  times  faster  in  water  than  in  castor  oil. 
The  point  which,  in  the  present  connection,  interests 
us  most  is  that  the  velocity,  other  things  being 
equal,  is  proportional  to  the  square  of  the  radius. 

To  fix  ideas  it  will  be  useful  to  consider  an  example 
in  figures,  say  a  gold  particle  of  i  p,  radius  or  2  n 


32     CONCLUSIONS  FROM  STOKES'  FORMULA. 

diameter.  Introducing  the  proper  values  (all  in 
centimetres,  grammes  and  seconds),  viz.,  r  =  io~4, 
s  =  19-3,  s'  (water)  =  i,  g  =  980,  rj  at  20°  C.  =  o-oi, 
we  find  the  velocity  of  the  particle  about  0-04  mm. 
per  second,  or  2-4  mm.  per  minute. 

This  is  a  considerable  speed  and  means,  in  other 
words,  that  such  a  suspension  of  gold  particles  would 
clear  at  the  rate  of  2-4  mm.  per  minute  from  the  top, 
and  would  be  clear  to  a  depth  of  about  14  cm.  at  the 
end  of  one  hour. 

Assuming  now  the  radius  to  be  i/ioo  of  that  just 
considered,  or  10  ^,  which  is  the  size  of  the  particles 
in  many  red  gold  sols,  the  velocity  would  be  1/10,000 
of  that  calculated.  This  makes  it  only  0-014  mm. 
per  hour,  or  about  10  mm.  in  one  month.  With 
particles  of  lower  specific  gravity  the  rate  of  settling 
would  be  proportionately  slower  :  with  a  specific 
gravity  of  3,  instead  of  19-3,  it  would  be  about  one- 
sixth  of  the  above  figure,  i.e.,  about  1-3  mm.  per 
month. 

This  little  calculation  shows  us  that,  with  particles 
of  ultra-microscopic  size,  a  suspension  may  appear 
very  stable  and  may  take  a  very  long  time  to  show 
marked  clearing  or  sedimentation.  At  the  same 
time,  disperse  systems  with  ultra -microscopic 
particles  differ  from  coarser  ones  in  their  liability, 
already  referred  to,  to  undergo  irreversible  transfor- 
mations. A  coarse  suspension  is  reversible  :  when 
it  has  settled,  however  long  the  process  may  take,  it 
can.be  restored  to  its  original  condition  by  mere 
shaking  or  agitation,  and  this  may  be  repeated 
indefinitely.  This  is  not  generally  possible  with  sols 
like  the  metal  sols  ;  when  these  have  coagulated,  the 
gel  can  hardly  ever  be  transformed  back  into  the 
original  sol  merely  by  mechanical  agitation.  We  are 
therefore  forced  to  the  conclusion  that  the  particles 
are  subject  to  other  influences  besides  those  of  gravity 
and  of  viscosity,  and  investigation  shows  this 


BROWNIAN   MOVEMENT.  33 

reasoning  to  be  correct,  inasmuch  as  the  particles  are 
in  violent  motion,  and  are  also  electrically  charged. 

The  motion  of  the  particles  is  the  most  striking 
feature  of  the  ultra-microscopic  picture.  It  is,  however, 
visible  even  with  much  larger  particles  and  ordinary 
illumination,  and  was  first  observed  (on  pollen  grains 
suspended  in  liquid)  by  Dr.  Robert  Brown,  the 
botanist,  after  whom  it  is  called  the  Brownian  move- 
ment. The  movement  is  composed  of  an  oscillating 
motion  of  the  particles  round  a  central  position,  and 
an  erratic  translatory  motion.  Description  is  rather 
inadequate,  but,  if  an  ultra-microscope  or  ultra-con- 
denser is  not  available,  it  may  be  seen  quite  well  in  a 
suspension  of  gamboge  (the  ordinary  water  colour)  with 
a  magnification  of  about  500  diameters  and  such  simple 
dark  ground  illumination  as  can  be  obtained  with  a 
central  stop  in  the  Abbe  or  achromatic  condenser. 

The  phenomenon  received  attention  from  a  number 
of  investigators,  including  Wiener,  Gouy,  Jevons 
Exner  and  Ramsay,  during  the  nineteenth  century. 
It  was  shown  that  particles  of  any  material  showed 
the  movement,  provided  they  were  sufficiently  small, 
that  it  decreased  with  increasing  size  and  became 
imperceptible  when  the  size  increased  above  about 
3  p,  diameter.  Various  suggested  causes,  such  as 
vibration,  convection  currents  due  to  changes  of 
temperature  or  concentration,  the  effect  of  illumina- 
tion, etc.,  were  gradually  eliminated  by  experiment, 
and  towards  the  end  of  the  century  the  opinion  gained 
ground  that  the  cause  of  the  movement  had  to  be 
sought  in  some  factor  inherent  in  the  liquid  state. 
The  invention  of  the  ultra-microscope  gave  a  great 
impetus  to  the  study  of  the  phenomenon,  as  the  very 
small  particles  revealed  by  it  for  the  first  time 
showed  such  a  vivid  motion  that  Zsigmondy  was 
inclined  to  look  on  it  as  something  differing  not  only 
in  degree,  but  also  in  its  nature  from  the  Brownian 
movement  as  known  up  to  then. 


34  SVEDBERG'S   WORK. 

Quantitative  relations  were  first  established  in 
1906,  both  by  experimental  investigation  and  by 
mathematical  treatment.  The  former  was  carried 
out  by  Svedberg,  who  allowed  sols  to  flow  through 
the  chamber  of  the  ultra-microscope  with  the  effect 
that  the  Brownian  movement  was  combined  with 
the  translatory  motion  of  the  liquid  as  a  whole,  the 
path  of  the  particle  appearing  as  a  wave  line.  Rela- 
tions could  then  be  established  between  the  ampli- 
tude and  the  wave-length,  both  being  measured  by 
the  eye -piece  micrometer.  The  former  is  the  maxi- 
mum deviation — more  correctly  its  projection  on  a 
plane  perpendicular  to  the  axis  of  the  microscope — 
of  the  particle  from  a  mean  position,  while  from  the 
wave-length  and  the  known  velocity  of  flow  through 
the  field  the  period,  i.e.,  the  time  taken  by  the 
particle  to  return  to  the  mean  position,  could  be 
deduced.  From  a  great  number  of  observations  on 
sols  with  different  disperse  phases  and  different 
dispersion  media  Svedberg  deduced  two  relations  : 
(i)  For  a  given  radius  the  amplitude  is  inversely  pro- 
portional to  the  viscosity  of  the  dispersion  medium  ; 
and  (2)  The  period  is  proportional  to  the  amplitude. 
In  symbols  these  relations  may  be  written  :— 

(i)     An  =  Cl  (2)     A  fP  =  C2 

where   A  -  =  amplitude,  P  =  =  period,   ?;  =  =  viscosity 
and  C1  and  C2  are  constants. 

In  1906  the  problem  was  treated  mathematically 
by  Einstein  and  by  v.  Smoluchowski  on  the  assump- 
tion that  the  movement  was  caused  by  the  impact 
of  the  molecules  of  the  dispersion  medium  on  the 
particles.  Their  results,  although  obtained  by 
different  methods,  are  identical  except  for  a  numerical 
constant.  Smoluchowski 's  formula  is  :-— 

42  _  64  P     RT 

~~  27  2  ^NTTr]  r 
in  which  A,  P  and  ?;  have  the  same  meaning  as  above  ; 


THEORY  OF  BROWNIAN  MOVEMENT.  35 

r  is  the  radius  of  the  particle,  while  R  is  the  gas 
constant,  T  the  absolute  temperature  and  N 
Avogadro's  number.  Einstein's  formula  has  the 
factor  i  instead  of  64/27. 

For  a  given  radius  r  and  temperature  T  the 
expression  on  the  right  hand  becomes  constant  with 
the  exception  of  r]  and  P,  and  by  a  simple  transfor- 
mation we  can  write  it  :— 

4A*n/P  =  C,. 

It  will  readily  be  seen  that  the  same  result  is  obtained 
by  multiplying  the  two  expressions  found  experi- 
mentally by  Svedberg  with  each  other. 

The  question  is  sometimes  asked  why  the  impacts 
of  the  liquid  molecules  do  not  balance  each  other, 
i.e.,  why  they  produce  any  visible  movement  at  all. 
The  answer  is,  of  course,  that,  when  the  particles  are 
small,  the  probability  of  the  impacts  exactly  balanc- 
ing in  a  given  short  time  is  also  small  ;  it  increases 
with  increasing  size,  with  which  the  Brownian 
movement  finally  becomes  imperceptible. 

Einstein's  and  v.  Smoluchowski's  deductions  lead 
to  a  view  which  has  been  very  clearly  put  by  Perrin  ; 
that  disperse  particles  differ  from  molecules  merely 
by  their  size  or,  in  other  words,  may  be  looked  upon 
as  very  large  molecules.  This  leads  to  several 
consequences  that  can  be  tested  experimentally  ; 
one  of  them  is  that  the  particles  in  a  disperse  system 
must,  under  the  influence  of  gravity,  arrange  themselves 
as  do  the  molecules  of  a  gas  in  the  same  conditions.  It 
is  well  known  that  the  density  of  a  gas,  i.e.,  the 
number  of  molecules  in  unit  volume,  varies  with  the 
height  according  to  an  exponential  law,  and  a 
similar  law  should  therefore  hold  good  for  the 
number  of  particles  at  different  heights  in  a  disperse 
system.  While,  however,  it  is  necessary  to  ascend 
to  considerable  heights  in  the  atmosphere  to  show 
a  marked  decrease  in  density  (i.e.,  pressure),  a 

3-2 


36  PERRIN'S   WORK. 

difference  in  concentration  should  be  demonstrable 
at  very  slight  differences  of  level  when  the  molecules 
are  of  enormous  size.  Perrin  examined  suspensions 
of  gamboge  particles  of  uniform  size,  and  counted 
the  particles  at  different  levels  in  a  layer  of  only 
0-12  mm.  total  thickness.  He  found  that  the 
number  actually  decreased  with  increasing  height 
according  to  an  exponential  law.  The  "  molecular 
weight  "  of  the  gamboge  particles  was  found  to  be 
about  3  x  io9. 

For  further  details  of  these  classical  researches  the 
reader  is  referred  to  Perrin 's  work,  which  has  been 
translated  into  English  by  Soddy  under  the  title, 
"  Brownian  Movement  and  Molecular  Reality." 

We  will  mention  only  one  further  result  of  these 
investigations  ;  they  may  be  used  for  calculating 
Avogadro's  number,  i.e.,  the  number  of  molecules 
contained  in  i  gramme-molecule  of  any  gas.  The 
value  found  by  Perrin,  7  x  io23,  agrees  well  with  the 
values  found  by  a  number  of  methods  based  on 
entirely  different  principles. 

The  view  that  suspensoid  particles  may  be  con- 
sidered as  very  large  molecules  leads  to  one  or  two 
further  conclusions,  which  are  in  complete  accord- 
ance with  experience.  Owing  to  the  high  "  mole- 
cular weight  "  and  the — generally — low  percentage 
concentration,  the  molecular  concentration  of  sus- 
pensoid sols  is  extraordinarily  small,  and  we  should, 
therefore,  not  expect  them  to  show  appreciable 
osmotic  pressures.  For  similar  reasons  we  must 
expect  very  low  diffusion  constants  ;  both  conclusions 
are  borne  out  by  experiment.  At  the  same  time  it 
is  obvious  that,  as  the  size  of  the  disperse  particles 
decreases  there  must  be  a  steady  increase  in  osmotic 
pressure,  for  a  given  amount  of  disperse  phase,  and 
a  steady  transition  to  molecular  or  "  true  "  solutions 
— even  though  every  step  of  this  transition  may  not 
be  capable  of  experimental  realization. 


ELECTRIC   CHARGE.  37 

It  is  evident  that  the  Brownian  movement, 
especially  with  particles  of  ultra-microscopic  size, 
is  a  factor  which  counteracts  sedimentation.  As  it  is 
not  confined  to  this  range,  but  occurs  with  particles 
that  separate,  it  is,  however,  not  the  only  one,  since 
it  is  not  affected  directly  by  factors  which  destroy  the 
stability  of  suspensoid  sols,  e.g.,  the  addition  to  them 
of  electrolytes.  A  second  factor  intimately  connected 
with  the  stability  of  sols  on  the  one  hand,  and  the  irre- 
versible character  of  the  transformations  they  undergo 
on  the  other,  is  the  electric  charge  on  the  particles. 

Although  the  electrical  properties  of  the  suspen- 
soids  have  probably  received  more  attention  than 
all  others  together,  the  origin  of  the  electric  charge 
is  still  a  subject  of  controversy,  which  will  be  more 
conveniently  discussed  when  we  have  become 
familiar  with  the  preparation  and  properties  of 
individual  sols.  Here  it  may  be  said  that  any  sub- 
stance in  contact  with  water  and  many  other  liquids 
assumes  an  electric  charge,  which  can  be  varied  both 
in  amount  and  in  sign  by  the  addition  of  electrolytes, 
and  may  become  zero  at  suitable  concentrations  of 
the  latter.  In  this  condition,  as  has  been  shown  by 
Hardy  and  by  Burton,  suspensoid  sols  are  particu- 
larly unstable  and  tend  to  precipitate  ;  the  electro- 
lyte concentration  at  which  the  disperse  phase  shows 
no  charge  has  been  called  by  Hardy  the  isoelectric 
point.  Anomalies,  viz.,  maximum  instability  when 
the  particles  are  not  electrically  neutral,  have  been 
observed  and  will  be  referred  to  again. 

However  obscure  the  origin  of  the  charge,  its 
existence,  and  that  of  the  opposite  charge  on  the 
dispersion  medium,  can  easily  be  demonstrated.  If 
an  electric  field  is  produced  in  a  disperse  system, 
whichever  phase  can  move  freely  will  move  towards 
the  electrode  having  the  opposite  sign  to  that  carried 
by  the  moving  phase.  Thus,  in  a  sol  the  particles 
will  travel,  while,  if  we  fix  the  disperse  phase,  say, 


38     DEMONSTRATING   ELECTRIC  CHARGE 

in  the  shape  of  a  porous  plug  in  a  tube  filled  with 
water,  the  liquid  will  flow.     The  latter  phenomenon, 


Fig.    5   —  U-TuBE      FOR 
CATAPHORESIS. 


FIG.  6.  —  SLIDE  FOR 
MICROSCOPIC  MEA- 
SUREMENT OF  CATA- 
PHORESIS. 


which  is  of  technical  interest,  is  called  electro- 
cndosmosis  ;  the  former,  which  is  employed  to  deter- 
mine the  sign  of  the  charge  on  suspensoid  particles, 


DEMONSTRATING  ELECTRIC  CHARGE.     39 

is  generally  called  cafaphoresis,  although  elect ro- 
phoresis  would  be  a  more  suitable  general  term. 

Cataphoresis  can  be  demonstrated  by  macroscopic 
as  well  as  by  microscopic  methods.  The  former 
consists,  in  principle,  in  placing  the  sol  to  be 
examined  into  the  bend  of  a  U-tube  (Fig.  5)  and 
filling  the  limbs  with  pure  water  or,  more  correctly, 
with  some  liquid  which  does  not  give  rise  to  a 
difference  of  potential  at  the  surface  of  contact  with 
the  sol.  Electrodes  dip  into  the  limbs  and,  when 
these  are  connected  to  a  source  of  current,  the 
particles  gradually  wander  into  the  water  surround- 
ing the  pole  of  opposite  sign,  so  that  negatively 
charged  particles  travel  to  the  anode,  and  positively 
charged  particles  to  the  cathode. 

The  microscopic  method  was  first  used  by  Cotton 
and  Mouton  and  permits  the  use  of  very  small 
volumes  of  liquid.  An  ordinary  microscope  slide 
(Fig.  6)  is  provided  with  a  pair  of  parallel  electrodes 
of  platinum  foil,  which  are  connected  by  suitable 
leads  to  a  couple  of  cells  or  accumulators.  A  drop 
of  the  liquid  under  examination  is  spread  on  the  slide 
so  as  to  be  in  contact  with  both  electrodes  and 
covered  with  a  cover  glass.  The  preparation  is 
illuminated  with  one  of  the  dark-ground  condensers 
previously  described  and  examined  under  the  micro- 
scope, when  the  particles  will  be  seen  to  travel 
towards  either  electrode. 

In  the  U-tube  it  will  be  noticed  that  the  boundary 
between  sol  and  clear  water  advances  parallel  to 
itself  towards  the  electrode,  which  shows  that  all 
particles  travel  with  the  same  speed.  If  disturbing 
factors  are  avoided,  the  same  observation  is  made  t  y 
the  microscopic  method.  This  speed  does  not  differ 
widely  even  in  different  sols,  and  lies,  generally 
speaking,  between  i  and  4  X  io~4  cm. /sec.  in  a 
potential  gradient  of  I  volt /cm.  These  values  do 
not  differ  greatly  from  the  velocities  of  the  slower  ions. 


CHAPTER   V. 

IN  the  preceding  chapter  we  have  denned  the 
suspensoids  as  systems  containing  the  disperse  phase 
as  solid  particles  below  a  certain  size,  in  constant 
movement  and  electrically  charged.  We  shall  now 
consider  a  number  of  typical  representatives  of  the 
class  in  some  detail  and  incidentally  attempt  to 
classify  the  methods  by  which  suspensoid  sols — all 
of  which  are  laboratory  products — are  obtained. 

Since  we  have  to  obtain  the  disperse  phase  in 
particles  of  a  size  lying  between  definite  limits,  we 
can  obviously  reach  the  desired  degree  of  dispersity 
from  either  side,  at  any  rate  in  principle,  i.e.,  we 
may  start  from  ions  and  molecules,  or  we  may  start 
from  material  in  a  coarser  state  of  division  and 
comminute  it  to  the  desired  extent.  This  distinction 
was  first  drawn  by  Svedberg,  who  divides  the  methods 
for  producing  sols  into  condensation  and  dispersion 
methods.  The  former  include  practically  all  methods 
which  employ  a  chemical  reaction  for  producing 
the  disperse  phase. 

The  sols  of  the  noble  metals,  and  more  especially 
of  gold,  are  among  the  most  typical  examples  of  the 
latter  procedure,  and  have  been  studied  exhaustively. 
To  prepare  gold  sol,  a  very  dilute  solution  of  either 
AuCl3  or  HAuCl4,  containing  i  part  in  10,000  or 
frequently  less,  and  either  exactly  neutralized  or 
made  slightly  alkaline,  is  reduced  by  one  of  a  large 
number  of  reducing  agents,  either  at  ordinary 
temperature  or  at  boiling  point.  Among  reducing 
agents  which  act  in  the  cold  are  hydrazine,  phenyl- 
hydrazine,  hydroquinone,  pyrocatechin,  pyrogallol, 


PREPARING   SUSPENSOIDS.  41 

etc.,  the  resulting  sols  being  blue  or  purple.  Reducing 
agents  which  are  used  in  hot  solution  include  ethyl 
alcohol,  formaldehyde,  tannin,  dextrin,  etc.  With 
suitable  procedure  and  precautions,  for  which  the 
works  on  practical  colloidal  chemistry  must  be 
consulted,  these  produce  beautiful  ruby  red  sols 
without  any  purple  tinge.  All  these  are  stable,  but 
some  of  them,  particularly  the  sols  reduced  by  ethyl 
alcohol  or  by  formaldehyde,  are  so  sensitive  to  low 
concentrations  of  foreign  electrolytes,  that  even  the 
small  quantities  of  glass  dissolved  from  ordinary 
glass  ware,  or  acid  from  the  laboratory  atmosphere, 
cause  a  change,  so  that  they  must  be  kept  in  well- 
stoppered  vessels  of  resistance  glass. 

Silver  sols  may  be  obtained  in  similar  fashion  by 
reducing  very  dilute  solutions  of  silver  nitrate,  made 
alkaline  with  ammonia  or  sodium  hydroxide,  with 
most  of  the  organic  reducing  agents  mentioned  above 
in  the  cold.  They  show  a  great  variety  of  colour; 
from  blue  (hydroquinone)  to  light  brown  with  a 
marked  greenish  tinge  in  reflected  light  (tannin). 
Of  great  interest  are  the  silver  sols  first  prepared  by 
Carey  Lea,  especially  on  account  of  the  very  high 
concentration  of  disperse  phase.  He  reduced  silver 
nitrate  with  mixtures  of  ferrous  sulphate  and  an 
alkaline  citrate  or  tartrate,  or  else  with  dextrine  in 
the  presence  of  caustic  alkali.  Blue  or  brown  pre- 
cipitates result,  which,  after  washing  with  dilute  salt 
solutions  or  with  alcohol  disperse  spontaneously  in 
water  to  form  sols  with  concentrations  of  10  per 
cent,  and  more. 

A  method  of  producing  silver  sols  which  possesses 
considerable  theoretical  interest  is  the  reduction  of 
solutions  of  silver  oxide  by  gaseous  hydrogen,  which 
has  been  exhaustively  studied  by  Kohlschutter. 
Hydrogen  is  bubbled  through  a  saturated  silver 
oxide  solution,  containing  some  oxide  in  excess  and 
maintained  at  the  optimum  temperature  of  55°  to 


\2  PREPARING   SUSPENSOIDS. 

60°  C.  A  yellow  sol  is  gradually  produced,  which 
still  contains  silver  oxide  as  well  as  metallic  silver. 
Kohlschiitter  found  that  the  reduction  took  place 
exclusively  at  the  wall  of  the  vessel  and  that  the 
material  of  the  latter  exerted  a  considerable  influence 
on  the  colour  of  the  sol.  In  platinum  vessels  no 
colloidal  silver  is  formed  at  all,  but  the  reduced 
metal  appears  in  shining  crystals  on  the  platinum 
surface.  The  oxide  still  remaining  in  a  sol  prepared 
in  a  glass  vessel  can  therefore  be  removed  by  con- 
tinuing the  treatment  with  hydrogen  in  platinum 
vessels.  The  interest  of  the  method  lies  in  the 
possibility  of  producing  a  sol  practically  free  from 
electrolytes,  while  it  will  be  readily  understood  that 
the  methods  described  so  far,  and  to  be  still  men- 
tioned, all  leave  electrolytes  resulting  from  the 
reaction  in  the  sol. 

Sols  of  the  metals  of  the  platinum  group  can  also 
be  obtained  by  direct  reduction  with  hydra  :ine,  etc., 
but  are  rarely  stable  without  certain  additions  which 
will  be  discussed  later.  Sols  of  mercury  appear  to 
have  been  made  for  the  first  time  by  Lottermoser, 
who  reduced  solutions  of  mercurous  nitrate  with 
stannous  nitrate. 

As  regards  non-metallic  elements,  tellurium  and 
selenium  sols  can  be  obtained  by  reducing  solutions 
of  the  dioxides  with  hydrazine  or  phenylhydrazine 
hydrochloride.  The  latter  sol  is  a  vivid  red.  The 
most  extensively  studied  sol,  however,  is  that  of 
sulphur  :  the  work  of  early  investigators  has  been 
mentioned  in  Chapter  I.  An  important  modern 
method  of  preparing  colloidal  sulphur  is  that  of 
Raffo  :  the  decomposition  of  sodium  thiosulphate 
by  concentrated  sulphuric  acid.  Some  of  the  sulphur 
liberated  by  the  reaction  is  coarse,  but  a  large  fraction 
is  colloidal,  and  can  be  separated  and  purified  by 
suitable  treatment.  This  method  has  been  developed 
by  Sven  Oden,  who  has  made  colloidal  sulphur  the 


PREPARING   SUSPENSOIDS.  43 

subject    of    probably    the    most    exhaustive    study 
devoted  to  a  single  sol  (Der  kolloide  Schwefel,  Upsala, 

19*3)  • 

Among  the  numerous  compounds  which  can  be 
obtained  in  the  suspensoid  state  the  sulphides  must 
be  particularly  mentioned.  Arsenic  trisulphide, 
which  is  easily  obtained — as  noticed  already  by 
Berzelius — by  passing  hydrogen  sulphide  into  a 
solution  of  arsenious  oxide,  has  been  the  subject  of 
the  classical  investigations  of  Linder  and  Picton. 
Mercuric  sulphide  can  be  obtained,  in  either  water 
or  alcohol  as  dispersion  medium,  by  passing  hydrogen 
sulphide  into  a  solution  of  mercuric  cyanide. 

Other  compounds  can  be  referred  to  only  very 
briefly.  Of  great  theoretical  interest  are  Lotter- 
moser's  investigations  on  silver  haloid  sols.  These 
can  be  obtained  by  double  decomposition  in  suitable 
concentrations  if  either  the  Ag  ion  or  the  halogen  ion 
is  kept  in  excess  throughout  the  reaction,  i.e.,  by 
pouring  a  certain  volume  of  dilute  AgNO3  solution 
into  a  somewhat  larger  volume  of  an  equivalent 
solution  of  KI  or  KBr,  or  vice  versa.  An  important 
difference  between  the  sols  resulting  from  these 
converse  procedures  will  be  discussed  later  on. 

The  condensation  methods,  of  which  a  fewexamples 
only  have  been  described,  have  certain  features  in 
common.  The  disperse  phase  is  produced  from — 
generally  speaking — very  dilute  solutions  of  the 
reaction  components,  and  in  practically  all  cases 
electrolytes,  in  correspondingly  low  concentrations, 
are  present  in  the  sol.  That  these  play  some  part 
beyond  that  of  unavoidable  by-products  is  proved 
by  the  observation,  which  can  be  made  with  most 
sols,  that  attempts  to  remove  the  electrolytes  com- 
pletely by  dialysis  lead  to  the  coagulation  of  the  sol. 
The  probable  cause  of  this  behaviour  will  be  con- 
veniently discussed  in  connection  with  the  electrical 
properties. 


44  THE   WORK   OF   v.    WEIMARN. 

The  mechanism  of  the  condensation  methods  has 
been  completely  elucidated  by  the  labours  of  P.  P. 
von  Weimarn.  Without  going  into  detail  or  repro- 
ducing the  mathematical  formulation  it  may  be  said 
briefly  that  he  treats  suspensoid  formation  as  a 
special  case  of  crystallization  from  supersaturated 
solution.  If  we  produce  an  insoluble  or,  strictly 
speaking,  very  slightly  soluble  substance,  it  will 
separate  in  particles  which,  according  to  v.  Weimarn, 
are  always  crystalline  whatever  their  size.  If  we 
wish  to  keep  the  latter  within  certain  limits,  two 
conditions  have  to  be  satisfied  ;  condensation  must 
begin  in  a  large  number  of  places,  and  the  growth  of 
the  nuclei  thus  formed,  which  necessarily  will  be  of 
molecular  dimensions  at  first,  beyond  the  desired 
limit  must  be  prevented.  Provided  a  disperse 
system  is  to  form  at  all,  more  reaction  product  must 
be  formed  than  can  exist  in  true  solution  in  the  par- 
ticular dispersion  medium  used  ;  the  ratio  of  this 
excess/solubility  settles  the  rate  of  condensation, 
which  varies  in  the  same  sense  as  the  ratio.  It  can, 
therefore,  be  increased  in  either  of  two  ways — by 
increasing  the  excess  or  by  reducing  the  solubility. 
If  the  latter  is  appreciable  (the  solubility  of  barium 
sulphate,  0-00024  gm-  in  I0°  c-c-  at  I8°  is  appreciable 
in  the  present  connection)  the  excess  must  be  large  ; 
in  that  case,  while  the  precipitate  is  still  highly 
disperse,  the  particles  are  so  numerous  and  close 
together  that  a  gel  is  formed.  If  the  excess  is 
smaller,  the  solubility  must  be  reduced  ;  in  the  case 
of  barium  sulphate  this  can  be  accomplished,  e.g.,  by 
adding  alcohol  to  the  reaction  mixture,  when  a  stable 
sol  of  barium  sulphate  can  be  obtained.  The  growth 
of  the  nuclei  again  depends  on  the  amount  present 
in  true  solution,  i.e.,  on  the  solubility  and  on  the 
rate  of  diffusion  ;  the  latter  depends  on  various 
factors,  including  the  viscosity  of  the  dispersion 
medium.  By  a  suitable  adjustment  of  these 


DISPERSION   METHODS.  45 

variables,  practically  any  substance  may  be  made  to 
form  a  sol ;  v.  Weimarn  and  his  school  have  proved 
this  possibility  experimentally  for  several  hundred 
substances. 

The  dispersion  methods,  which  start  from  the 
material  of  the  disperse  phase  in  bulk  or  in*a  state 
of  division  obviously  coarser  than  the  colloidal,  have 
been  divided  by  Svedberg  into  chemical  and 
mechanical  on  the  one  hand,  and  electrical  on  the 
other.  Instances  of  the  former  occur  frequently  ; 
many  cases  are  familiar  to  the  analyst  in  which  a 
precipitate,  on  being  gradually  washed  free  from  the 
solution  in  which  it  was  formed,  becomes  sufficiently 
disperse  to  "  pass  through  the  filter."  Some  metallic 
sulphides  are  conspicuous  examples.  In  other  cases 
precipitates  can  be  dispersed  by  the  action  of  electro- 
lytes, a  procedure  already  employed  by  Graham,  and 
called  by  him  "  Peptization."  A  good  example  is 
the  preparation  of  cadmium  sulphide  sol.  The 
sulphide  is  precipitated  by  treating  an  ammoniacal 
solution  of  cadmium  sulphate  with  ammonium  or 
hydrogen  sulphide,  thoroughly  washed  and  suspended 
in  water.  If  hydrogen  is  now  passed  through  the 
suspension,  the  flocculent  precipitate  gradually 
breaks  up,  the  liquid  becomes  milky  and  finally 
perfectly  clear  in  transmitted  light,  the  colour  being 
a  golden  yellow.  Mercuric  sulphide  sol  may  be 
prepared  in  a  similar  manner. 

A  method  involving  both  chemical  and  mechanical 
treatment  is  that  developed  by  Kuzel  for  preparing 
sols  of  various  metals,  such  as  uranium,  tungsten, 
vanadium,  etc.  The  material  is  first  ground  as  finely 
as  possible,  and  is  then  digested  alternately  with 
dilute  acid,  water  and  dilute  alkali.  After  a  number 
of  treatments  it  disperses  spontaneously  in  distilled 
water.  The  tungsten  obtained  from  such  sols  by 
coagulation  was  used  for  making  "  squirted  "  fila- 
ments for  incandescent  lamps  before  the  methods  of 


46  DISPERSION   METHODS. 

drawing  tungsten  wire,  which  are  now  employed, 
had  been  developed. 

In  some  cases  an  appreciable  proportion  of 
material  may  be  reduced  to  colloidal  sizes  merely 
by  prolonged  grinding.  The  requisite  degree  of 
fineness  is  best  secured  by  "  diluting  "  the  material 
to  be  dispersed  with  some  other  solid,  which  can 
be  removed  afterwards  by  an  indifferent  solvent 
other  than  the  dispersion  medium  to  be  used  even- 
tually. This  method  has  not  yet  been  studied 
extensively  (cf.  The  Svedberg,  "  The  Formation  of 
Colloids,"  J.  and  A.  Churchill,  1921). 

Among  the  electric  dispersion  methods  the  disinte- 
gration in  the  electric  arc  is  by  far  the  most 
interesting.  This  was  first  studied  by  Bredig  (1898) 
who  found  that,  when  a  (continuous  current)  arc 
was  produced  under  water  between  electrodes  of  the 
noble  metals,  the  liquid  became  deeply  coloured  and 
showed  all  the  properties  of  colloidal  solutions  as 
then  known.  Some  metals  only  gave  coarse  disper- 
sions, while  others  were  oxidised,  hydroxide  sols 
being  formed.  In  most  cases  a  very  slight  concen- 
tration of  alkali  in  the  water  is  favourable  or  even 
indispensable  if  stable  sols  are  to  result. 

The  dispersion  by  the  electric  arc  was  further 
developed  and  investigated  exhaustively  by 
Svedberg.  Instead  of  the  continuous  current  he 
used  various  other  types  of  discharge,  e.g.,  oscillating, 
employed  electrodes  of  metals  which  do  not  disinte- 
grate, such  as  iron  or  aluminium,  between  which  the 
metal  to  be  dispersed  was  suspended  as  foil  or 
fragments,  and  carried  out  the  dispersion  in  a 
number  of  organic  liquids.  He  succeeded  in  pro- 
ducing sols  of  the  alkali  metals  and  the  metals  of  the 
alkaline  earths  at  very  low  temperatures.  For 
details  of  these  researches  and  their  bearing  on  the 
theory  of  sol  formation  the  reader  is  referred  to 
Svedberg's  book  quoted  above. 


DISPERSION   METHODS.  47 

While  the  definition  oi  the  condensation  methods 
is  unambiguous,  it  is  by  no  means  certain  that  the 
dispersion  methods  really  lead  in  o»e  step  to  the 
formation  of  colloidal  particles.  This  question  has 
received  a  good  deal  of  attention  as  far  as  disin- 
tegration by  the  arc  is  concerned,  and  there  is  strong 
evidence  that  the  process  may  consist,  at  any  rate 
in  part,  in  the  formation  of  metallic  vapour  and  its 
subsequent  condensation  to  colloidal  particles.  The 
actual  mechanism  of  peptization  is  also  in  need  of 
elucidation. 


CHAPTER  VI. 

THE  suspensoid  sols,  whatever  the  disperse  phase, 
show  a  reasonable  uniformity  in  their  behaviour,  so 
that  it  is  possible  to  summarize  their  more  important 
properties.  The  first  to  claim  our  interest  are  the 
fundamental  properties  of  liquids,  -surface  tension 
and  viscosity.  The  former  does  not  differ 
appreciably  from  that  of  the  dispersion  medium. 
As  regards  the  latter,  the  concentration  of  disperse 
phase  is,  generally  speaking,  so  low  that  the  viscosity 
of  a  suspensoid  sol  is  very  slightly  higher  than  that 
of  the  dispersion  medium,  so  that  quantitative 
investigation — if  the  differences  are  to  be  decidedly 
in  excess  of  the  experimental  errors — is  confined  to 
the  sols  which  can  be  obtained  in  exceptional 
concentration,  or  to  suspensions  of  particles  of  much 
larger  than  colloidal  size.  Among  the  former  must 
be  mentioned  Sven  Oden's  sulphur  sols  :  two  series 
of  viscosity  measurements  given  in  his  monograph 
on  "  Colloidal  Sulphur,"  already  referred  to,  are  of 
special  interest,  as  they  were  carried  out  on  sols 
having  particles  of  very  different  size,  viz.,  about 
100  fjifji  in  A  and  about  10  ^  (estimated)  in  B.  The 
table  gives  the  percentage  by  volume  of  disperse 
phase  and  the  relative  viscosities,  that  of  water  at 
the  same  temperature  being  taken  as  unit}7.  The 
results  are  plotted  in  Fig.  7. 

Per  cent,  of 
disperse  phase.  r? 4  ??# 

5  ..         ..     1-20          1-30 

10  . .         -. .     1-50          172 

15  .  .  .  .       2-00  2-38 

20  ..  ..       275  3-63 


VISCOSITY   OF   SUSPENSIONS. 


49 


It  will  be  seen  that  the  sol  with  the  smaller  particles 
has  the  higher  viscosity  throughout,  and  that  in  both 
sols  the  viscosity  increases  more  rapidly  than  the 
concentration  of  disperse  phase.  The  latter  result 
was  also  obtained  by  Humphrey  and  Hatschek  in 
a  series  of  measurements  on  a  very  coarse  system 
(rice  starch  suspended  in  a  mixture  of  carbon 
tetrachloride  and  toluene  of  the  same  density)  carried 
out  at  very  low  rates  of  shear.  The  viscosity 
increased  more  rapidly  than  the  percentage  of 
disperse  phase,  and,  in  addition,  was  found  to  vary 


10 


20  % 


FIG.  7.— VISCOSITY  OF  SULIHUR  SOLS  OF 
DIFFERENT  DEGREES  OF  . DlSPERSITY . 

with  the  rate  of  shear,  so  that  the  complete  viscosity 
values  lie  on  a  surface,  the  other  co-ordinates  of 
which  are  the  concentration  of  disperse  phase  and 
the  shear  gradient. 

The  first  mathematical  treatment  of  the  problem 
was  given  by  Einstein  (1906),  who  deduced  the 
viscosity  of  a  suspension  of  rigid  spheres  suspended 
in  a  liquid  from  the  fundamental  equations  of 
hydrodynamics.  If  we  call  rj  the  viscosity  of  the 
dispersion  medium,  rf  that  of  the  disperse  phase,  and 


50  THEORETICAL  'FORMULA. 

K  the  ratio  :  volume  of  disperse  phase/total  volume 
Einstein's  formula  is 

?'=  77  (i  +  '2-5*0- 

This  means  that  the  viscosity  increases  in  linear 
ratio  with  the  percentage  of  disperse  phase  and,  since 
the  radius  does  not  appear  in  the  equation,  that  it  is 
independent  of  the  size  of  the  particles. 

For  low  concentrations  and  microscopic  particles 
the  formula  holds  approximately,  as  was  first  shown 
by  Bancelin  (1911)  on  suspensions  of  gamboge 
particles.  For  sols  and  especially  for  higher  concen- 
trations, however,  as  appears  from  Oden's  and  other 
measurements,  the  increase  is  much  more  rapid  than 
linear  and  is  not  independent  of  the  size  of  the 
particles.  A  discussion  of  the  causes  of  this  discre- 
pancy is  beyond  the  scope  of  this  work,  but  one 
difficulty  in  the  way  of  applying  any  mathematical 
deduction  rrfay  be  pointed  out.  The  latter  must 
inevitably  rest  on  ratios  of  volumes,  whereas  we 
know  with  certainty  only  the  weight  of  disperse 
phase  :  the  volume  calculated  therefrom  on  the 
assumption  that  the  density  of  the  particles  is  the 
same  as  that  of  the  material  in  bulk  need  not  be, 
and  probably  is  not,  correct.  In  addition,  there  is 
a  strong  probability  that  the  particles  may  carry 
with  them  envelopes  of  dispersion  medium,  which 
would  contribute  to  the  effective  volume. 

Quite  recently  (1920)  an  attempt  has  been  made 
by  R.  W.  Hess  to  deduce  synthetically  a  formula 
for  the  viscosity  of  a  suspension  of  rigid  particles. 
He  arrives  at  the  following  expression  :— 


in  which  the  symbols  have  the  same  meaning  as 
above,  and  a  is  a  "  supplementary  factor  "  always 
>  i,  which  varies  with  the  shape,  size  and  number 
of  particles,  and  piobably  with  the  rate  of  shear. 


OPTICAL  PROPERTIES.  51 

The  formula  is  not  linear  and,  with  a  variable  a, 
can  be  made  to  fit  any  viscosity-concentration  curve 
very  closely  ;  whether  definite  relations  between  the 
variables  mentioned  and  the  value  of  a  can  be 
deduced,  remains  to  be  seen.  Hess  himself  has 
tested  the  formula  on  suspensions  of  red  blood 
corpuscles,  which  are  not  only  large  (7-5  M  maximum 
diameter)  and  of  peculiar  shape,  but  also  easily 
deformable.  A  recent  investigation  by  Liiers  and 
Schneider  (Koll.-Zeitschr.  27,  273,  1920)  shows  that 
the  viscosity  of  a  given  suspension — flour — can  be 
represented  by  the  formula  with  a  reasonably  con- 
stant value  of  a. 

The  temperature  coefficient  of  the  viscosity  of 
suspensoid  sols  or  coarser  suspensions  is  practically 
that  of  the  dispersion  medium,  in  striking  contrast 
with  most  emulsoid  sols. 

As  regards  the  optical  properties,  we  are  already 
familiar  with  the  Tyndall  cone,  which  is  visible  in 
practically  all  suspensoid  sols.  It  may  appear 
either  merely  turbid,  or  show  a  colour  different  from 
that  of  the  sol  in  transmitted  light,  e.g.,  the  cone 
appears  greenish  in  some  red  gold  or  brown  silver 
sols.  The  light  emitted  by  the  Tyndall  cone  is 
plane  polarized,  as  can  easily  be  demonstrated  by 
observing  it  with  a  suitable  analyser,  e.g.,  a  Nicol 
prism.  The  percentage  of  light  polarized  depends 
on  the  size  of  the  particles  and  increases  as  they 
become  smaller.  The  position  of  the  plane  of 
polarization  varies  with  the  nature  of  the  disperse 
phase,  more  particularly  with  its  electrical  conduc- 
tivity :  it  is  perpendicular  with  the  axis  of  the  beam 
for  dielectrics  and  oblique  for  metals. 

The  most  striking  optical  property  of  sols  is  their 
colour.  In  many  cases  this  is  more  or  less  the  same 
as  that  of  the  disperse  phase  in  coarser  states  of 
division,  for  instance,  in  the  sulphide  sols.  There 
is  usually  no  change  of  colour  on  coagulation  in  this 

4-2 


52  COLOUR  OF  SOLS. 

type,  only  an  increasing  turbidity  ;  nor  do  sols  in 
which  the  size  of  the  particles  has  been  varied  by 
known  methods  differ  appreciably  in  colour.  It  has, 
however,  been  shown  by  Auerbach  (Koll.-Zeitschr., 
27,  223, 1920),  that  a  non-metallic  sol,  that  of  sulphur, 
may  show  a  great  range  of  colours  in  suitable 
conditions.  *  A  dilute  solution  of  sodium  thiosulphate 
is  decomposed  by  phosphoric  acid,  the  concentra- 
tions being  so  adjusted  that  the  whole  process  takes 
15  to  25  minutes  to  complete  itself.  During  that 
time  the  colour  of  the  mixture  in  transmitted  light 
changes  from  a  pale  yellow  through  orange,  red  and 
purple  to  a  pure  blue,  the  change  in  colour  being 
due  apparently  merely  to  the  growth  of  the  sulphur 
particles.  No  other  instance  of  a  sol,  in  which  the 
disperse  phase  is  a  dielectric,  showing  polychromy 
has  so  far  been  described. 

In  metal  sols  the  colour  may  vary  very  consider- 
ably :  thus,  gold  sols  may  be  green  (unstable),  blue, 
violet,  purple  to  bright  ruby  red.  The  red  and 
purple  sols  turn  blue  on  addition  of  electrolytes,  and 
eventually  deposit  a  blue  sediment  :  the  colour 
change  is  accompanied  by  an  aggregation  of  the 
particles  into  larger  complexes.  A  blue  gold  sol 
prepared  directly,  say  by  reduction  with  hydrazine 
or  hydroquinone,  has  not  necessarily  larger  particles 
than,  say,  the  coagulum  of  a  purple  sol  prepared  by 
another  method,  and  the  question  of  colour  in  this 
case  is  complicated  by  the  possibility  of  incomplete 
reduction  ;  in  other  words,  the  disperse  phase  may 
not  be  metal,  but  a  mixture  of  metal  and  oxide,  or  of 
several -oxides.  For  gold  this  may  be  easily  demon- 
strated as  follows  :  a  dilute  solution  of  gold  chloride 
(1/20,000)  containing  a  little  dextrin  is  reduced 
with  a  few  drops  of  dilute  hydroquinone  solution, 
when  a  beautiful  dark  blue  sol  is  formed.  If  this 

*  A  similar  result  had  teen  obtained  earlier  by  Keen  and 
Porter  (Proc.  Royal  Soc.,  A,  Vol.  89,  372,  1914). 


COLOUR  OF  SOLS.  53 

is  now  heated  to  boiling  and  a  little  caustic  alkali 
is  added,  the  dextrin  completes  the  reduction  and 
the  sol  turns  red.  Of  course,  this  red  sol  can  be 
obtained  directly  by  reduction  with  dextrin  and 
alkali,  and  the  colour  is  very  much  the  same  in  both 
cases. 

Silver  sols  may  also  be  obtained  in  a  great  range 
of  colour,  from  blue  through  purple,  and  red  to  pale 
yellow.  The  same  possibility,  incomplete  reduc- 
tion, has  to  be  borne  in  mind  in  this  case  too. 

We  now  proceed  to  consider  the  most  striking — 
and  most  thoroughly  investigated— property  of 
suspensoid  sols,  their  behaviour  towards  electro- 
lytes, to  which  a  special  chapter  must  be  devoted. 


CHAPTER  VII. 

As  stated  in  Chapter  IV.,  the  particles  of  a  suspen- 
soid  sol  are  electrically  charged,  and  this  charge  in 
some  way  determines  the  stability  of  the  sol,  since 
on  its  being  neutralized  by  suitable  means,  e.g.,  the 
addition  of  electrolytes,  the  sol  becomes  unstable, 
and  the  disperse  phase  is  eventually  precipitated. 
The  mechanism  of  this  stabilizing  action  is  not  by, 
any  means  clear,  but  there  is  no  doubt  whatever  of 
its  existence.  If  a  sol  is  observed  in  the  ultra- 
microscope,  no  collisions  between  particles  will  be 
noticed  notwithstanding  their  vigorous  motion  ;  on 
addition  of  an  electrolyte,  however,  the  particles 
will  be  seen  to  unite  into  larger  aggregates.  A  very 
considerable  amount  of  research  has  been  devoted 
to  the  determination  of  the  minimum  concentra- 
tions of  different  electrolytes  which  produce  coagula- 
tion in  a  given  sol.  The  first  thing  to  be  noted  is 
that  coagulation  has  to  be  defined  arbitrarily  or 
conventionally,  either  as  a  colour  change  where  this 
occurs,  or  as  incipient  or  complete  sedimentation  of 
the  disperse  phase,  within  a  specified  period  of  time. 
Under  strictly  uniform  conditions  comparable  results 
can  be  obtained,  and  the  pioneer  work  of  Schultze, 
Linder  and  Picton,  and  Hardy  may  be  summarized 
as  follows  :— 

Coagulation  is  chiefly  determined  by  that  ion  of 
the  electrolyte  which  carries  a  charge  opposite  to 
that  on  the  disperse  phase.  If  the  latter,  as  in  the 
majority  of  the  suspensoid  sols,  is  negatively  charged, 
the  determining  ion  is  therefore  the  cation  of  the 
electrolyte. 

The  electrolyte  concentration  necessary  to  produce 


EFFECT   OF  VALENCY.  55 

coagulation  is  the  lower  the  higher  the  valency  of 
the  active  ion.  It  decreases  much  more  rapidly  than 
the  valency  increases. 

A  certain  minimum  concentration  of  electrolyte 
is  necessary  to  produce  coagulation  at  all. 

The  coagulating  ion  is  found  in  the  coagulum, 
equivalent  amounts  of  different  ions  being  carried 
down  by  it. 

To  give  some  idea  of  the  general  order  of  magni- 
tude of  the  concentrations  of  different  electrolytes, 
a  few  selected  results  obtained  by  Freundlich  with 
an  arsenious  sulphide  sol  are  here  given  :  the  electro- 
lyte concentrations  are  given  in  millimoles  per  litre, 
and  refer,  of  course,  to  the  mixture  sol  -f-  electrolyte  :— 

KC1     49-5          MgCl2  0717       A1C13         0-093 

NaCl    51-0          CaCl2  0-649       A1(NO3)3  0-095 

LiCl     58-5          Ba(NO3)2    0-687 

It  will  be  noticed  that  (i)  ions  of  the  same  valency 
coagulate  in  approximately  the  same  concentration, 
and  (2)  the  concentration  of  univalent  ion  required 
to  produce  coagulation  is  roughly  70  times  tnat  of 
bivalent,  and  560  times  that  of  trivalent  ion.  If  we 
call  the  coagulating  concentrations  of  the  three  ions 
respectively  C1;  C2  and  C3,  where  the  index  represents 
the  valency,  we  can  therefore  write  approximately  :— 

Cj.  :  C2  :  C3  =  8-33  :  8-32  :  8-3 


An  empirical  relation  of  this  kind—  the  numerical 
constant,  of  course,  differs  considerably  from  one  sol 
to  another  —  had  been  found  by  Schultze  and  by 
Hardy  :— 

Cl  :  C2  :  C4  =  k*  :  k*  :  k 

Whetham  investigated  the  problem  mathemati- 
cally in  1899  and  deduced  a  formula  similar  to  the 
above  from  kinetic  considerations. 


ANOMALOUS   IONS. 


This  comparative  simplicity  is,  however,  found  to 
prevail  only  when  suitatly  chosen  values  are  com- 
pared. Considerat le  div  ergences  manifest  themselves 
when  the  field  of  investigation  is  extended.  Ions 
of  the  same  valency  and  combined  with  the  same  anion 
differ  very  considerably  in  their  coagulating  power 
It  was  found  at  an  early  date  that  hydrogen  ion— 
to  confine  the  comparison  to  cations  for  the  moment 
—was  effective  in  lower  concentrations  than  other 
cations  combined  with  the  same  anion,  i.e.,  that  acids 
coagulated  in  lower  concentrations  than  their  salts 
with  univalent  cations.  A  few  comparative  values 
determined  by  different  observers  are  given  to 
illustrate  this  point  :  — 

Mastic  sol  Gold  sol  As2S8  sol 

(Hardy).  (Hardy).  (Freundlich). 

HC1         0-004  HC1       0-008       HC1      0-031 

NaCl        0-12  NaCl      0-013       NaCl     0-051 

BaCl2      0-022 

Hardy's  data  are  given  in  gm.  equivalents  per  litre, 
and  Freundlich's  figures  have  been  reduced  to  the 
same  unit.  In  all  three  cases  HC1  coagulates  in 
considerably  lower  concentration  than  does  NaCl  ; 
moreover,  the  mastic  sol  is  coagulated  by  H  ion  in 
lower  concentration  than  by  the  bivalent  Ba  ion.  The 
latter  anomaly  is  ty  no  means  isolated. 

Organic  cations,  which  were  first  investigated 
extensively  by  Freundlich,  also  coagulate  in  much 
lower  concentrations  than  inorganic  ions  of  the  same 
valency.  Three  of  Freundlich's  values  for  the  alkali 
chlorides  are  repeated  below,  and  the  coagulation 
concentrations  for  three  chlorides  with  organic 
univalent  cations  are  given  for  comparison  :— 

As2S3  sol  as  above. 

NaCl     . .     51-0       Aniline  chloride  . .          .  .  2-52 

KC1-      .  .     49*5       Para-chloraniline  chloride  1-08 

LiCl       . .     58-5       Morphine  chloride  .  .  0-425 


EFFECT   OF  ANION.  57 

It  will  again  be  noticed  that  the  last-named 
chloride  coagulates  in  a  concentration  appreciably 
lower  than  even  that  of  the  chlorides  with  bivalent 
cations. 

If  we  confine  ourselves  to  the  consideration  of  any 
one  cation,  we  fj  nd  that  the  concentration  required 
to  coagulate  a  given  sol  varies  very  markedly 
according  to  the  nature  of  the  anion  with  which  it  is 
combined.  The  following  figures  found  by  Freund- 
lich  with  the  arsenious  sulphide  sol  already  referred 
to  show  the  effect  of  the  anion  very  strikingly  : — 

I/3K  Citrate  .  .          .  .  2/0 

KC2H3Oa no 

KCH02        86 

i/2K2S04 65-5 

KN03          <o-o 

KC1  . .          . .          . .          . .       49-5 

It  is  obvious  that  the  anion  exerts  some  effect  on 
the  phenomenon,  which  is  apparently  antagonistic  to 
that  of  the  cation,  and  does  not  depend  very  clearly 
on  valency.  Whatever  it  is,  it  cannot  be  separated 
from  that  of  the  cation,  and  this  is  probably  one  of 
the  reasons  why  attempts  to- deduce  a  general  valency 
rule,  in  which  the  active  ion  alone  is  taken  into 
account,  must  fail.  A  further  reason  for  this  failure 
is  the  existence  of  obvious  specific  differences  between 
various  types  of  cations.  These  make  comparison 
between  ions  of  the  same  valency  impossible  unless 
they  are  closely  related.  For  ions  permitting  such 
a  comparison  and  combined  with  the  same  ion,  the 
valency  rule  is  a  useful  guide,  though  for  any  given 
sol  it  will  still  require  verification  in  detail. 

Two  points  of  interest  have  not  been  touched  on  so 
far  :  the  effect  of  the  concentration  and  of  the  degree 
of  dispersity  of  the  sol  on  the  electrolyte  concentra- 
tion necessary  to  produce  coagulation.  As  regards 


58  POSITIVE  SOLS. 

the  former,  information  is  scanty,  being  derived 
principally  from  some  experiments  by  Freundlich, 
who  finds  that  the  electrolyte  concentration  increases 
with,  and  is  roughly  proportional  to,  the  concentration 
of  disperse  phase.  The  second  point  has  been  eluci- 
dated chiefly  by  Sven  Oden,  who  devised  methods 
of  obtaining  sulphur  sols  with  particles  of  uniform 
size,  which  could  be  varied  within  wide  limits.  (In 
sols  prepared  in  the  usual  way,  and  without  subse- 
quent treatment,  the  particles  are  generally  of 
different  sizes.)  Oden  found  that  for  equal  concen- 
tration by  weight  of  disperse  phase,  the  electrolyte 
concentration  necessary  for  coagulation  increased  with 
decreasing  size  of  the  particles — in  other  words,  the 
higher  the  degree  of  dispersity  the  greater  the 
stability  of  the  sol. 

We  have  so  far  referred  only  to  negatively  charged 
particles  and  to  cations.  It  follows  by  analogy  that, 
with  positively  charged  particles,  the  active  ion 
should  be  the  anion,  and  that,  therefore,  the  valency 
of  the  latter  should  be  the  factor  determining  the 
electrolyte  concentration.  The  principal  sols  in 
which  the  disperse  phase  is  positively  charged  are— 
apart  from  a  few  dyes  like  Night  Blue- — the  hydroxide 
sols,  which  we  shall  treat  separately  for  certain 
reasons.  Some  of  them,  however,  show  distinct 
suspensoid  character,  and  behave  towards  electro- 
lytes in  the  manner  just  anticipated,  as  will  be  seen 
from  the  following  concentrations  required  to  coagu- 
late ferric  hydroxide  sol,  also  determined  by  Freund- 
lich :  the  concentrations  are  given  in  millimoles  per 
litre  :- 

KG1   .  .          .  .  '9-03  K2SO4  .  .  0-204 

KNO3           . .  11-9  MgSO4  .  .  0-217 

i/2BaC!2       .  .       9-64  K2Cr2O7  .  .  0-194 
i/2Ba(OH)2..       0-42 

It  will  be  noticed  that  the  concentration  depends 


COAGULATION   VELOCITY.  59 

principally  on  the  anion,  that  the  bivalent  anions 
coagulate  in  much  lower  concentration  than  the 
univalent  ones,  and  that  among  the  latter  the  OH 
ion  again  occupies  an  exceptional  position,  as  does 
the  H  ion  among  the  cations. 

Electrolyte  coagulation — which  as  far  as  the  results 
quoted  hitherto  go  may  be  loosely  denned  as  agglo- 
meration of  the  particles  into  aggregates  of  sufficient 
size  to  show  sedimentation  within  a  limited  time — 
is  a  process  requiring  time,  and  recent  research  has 
been  devoted  to  studying  the  velocity  of  the  process. 
We  can  only  describe  very  briefly  the  first  of  these 
investigations,  carried  out  by  Paine  (1912)  with  a 
copper  oxide  sol  obtained  by  Bredig's  method  and 
carrying  a  positive  charge.  Very  rapid  coagulation 
and  settling  could  be  produced,  after  the  addition 
of  electrolyte,  by  heating  the  mixture  for  a  short 
time.  The  procedure  adopted  accordingly  consisted 
in  adding  to  a  given  volume  of  sol  known  amounts  of 
electrolyte,  removing  samples  at  known  intervals, 
heating  these  to  produce  sedimentation  and  deter- 
mining the  amount  of  copper  still  remaining  in  the 
clear  supernatant  liquid.  The  principal  results  are 
as  follows  :  (i)  there  is  a  latent  period  after  the 
addition  of  electrolyte  during  which  no  coagulation 
occurs  ;  (2)  coagulation,  once  begun,  proceeds  first 
rapidly  and  then  with  decreasing  velocity  ;  (3)  the 
velocity  increases  with  the  electrolyte  concentration  ; 
and  (4)  for  two  different  electrolyte  concentrations, 
the  times  required  to  reach  the  same  stages  in  coagu- 
lation are  in  constant  ratio,  i.e.,  if  the  electrolyte 
concentration  A  takes  twice  as  long  as  B  to  coagulate, 
say,  25  per  cent,  of  dispersed  phase,  A  will  also  take 
twice  as  long  as  B  to  coagulate  50  or  75  per  cent.,  etc. 
The  results  are  shown  graphically  in  Fig.  8,  in  which 
the  amount  of  copper  still  remaining  in  the  sol  is 
plotted  as  ordinate  against  the  time  as  abscissa. 
Two  curves  are  given,  for  two  electrolyte  concentra- 


6o 


COAGULATION  VELOCITY. 


tions,  and  the  statement  under  (4)  can  be  expressed 
thus  :  the  abscissae  of  pairs  of  points  on  the  two 
curves  which  have  the  same  ordinate  are  in  constant 
ratio  : 

AB/AC  =  A'B'/A'C'  =  A"B"/A"C". 

These  results  have  been  generally  confirmed  by 
subsequent  investigations,  among  which  those  by 
Freundlich  and  by  Zsigmondy,  with  various  colla- 


FIG.  8. — CURVES  ILLUSTRATING  RELATION  BETWEEN  COAGU- 
LATION VELOCITY  AND  ELECTROLYTE  CONCENTRATION. 

borators,  must  be  mentioned.  The  problem  of 
coagulation  velocity  has  also  been  treated  mathe- 
matically by  v.  Smoluchowski,  his  theoretical  con- 
clusions being  in  good  agreement  with  Zsigmondy's 
experimental  results.  It  is  worth  mentioning  that 
the  mathematical  treatment  involves  no  assump- 
tions, and  throws  no  light,  on  the  actual  process  of 
the  removal  of  the  electric  charge,  but  starts  from 
electrically  neutral  particles. 

A  brief  statement  of  the  theories  which  have  up 
to  the  present  been  propounded  to  account  for  the 


ORIGIN   AND   REMOVAL   OF   CHARGE.    61 

origin  of  the  electric  charge,  as  well  as  for  the 
mechanism  of  its  neutralization,  will  follow  more 
conveniently  when  the  electrical  properties  of  emul- 
soid  sols  and  the  phenomenon  of  absorption  have 
been  discussed. 


CHAPTER  VIII. 

• 

ONLY  one  means  of  removing  the  electric  charge 
on  suspensoid  particles,  viz.,  the  addition  of  electro- 
lytes to  the  sol,  has  been  discussed  so  far.  Since  sols 
with  both  negative  and  positive  charges  on  the  dis- 
perse phase  are  known,  the  idea  will  readily  suggest 
itself  that  neutralization  of  the  charges,  followed  by 
coagulation,  may  be  brought  about  by  mixing  sols 
with  opposite  charges.  Such  a  mutual  coagulation 
(of  certain  dyes)  was  first  observed  by  Linder  and 
Picton  (1897),  who  ascertained  that  the  sols  which 
precipitated  each  other  carried  charges  of  opposite 
signs.  Similar  observations  wrere  made  by  Lotter- 
moser  (1901)  with  various  inorganic  sols.  Biltz 
investigated  the  phenomenon  quantitatively  and 
found  that,  when  the  ratio  of  the  volumes  mixed  was 
kept  within  certain — fairly  narrow — limits,  complete 
coagulation  of  both  sols  took  place  ;  if  either  sol  was 
in  excess  of  this  optimum  ratio,  precipitation  was 
incomplete  or  did  not  take  place  at  all.  If  these 
uncoagulated  mixtures  are  submitted  to  cataphoresis, 
they  are  found  to  carry  the  same  charge  as  the  sol 
which  was  added  in  excess.  If  two  coloured  sols,  e.g., 
gold  and  ferric  hydroxide,  coagulate  each  other  com- 
pletely, the  supernatant  liquid  is  perfectly  colourless. 
An  electric  charge  of  course  exists  on  the  contact 
surface  of  any  substance  with  a  liquid,  though  it  does 
not  exert  perceptible  effects  unless  this  surface  is 
large.  It  attains  a  sufficient  magnitude  in  porous 
materials,  such  as  filter  paper,  which  becomes 
negatively  charged  in  contact  with  water.  If  a  strip 
of  filter  paper  is  partly  immersed  in  a  positive  sol, 


PROTECTIVE   COLLOIDS.  63 

the  disperse  phase  is  therefore  coagulated  in  the  pores, 
and  pure  dispersion  medium  rises  in  the  strip  by 
capillarity.  For  the  same  reason  positive  sols  cannot 
be  filtered  satisfactorily  through  filter  paper. 

Mixtures  of  suspensoid  sols  with  sols  of  marked 
emulsoid  character  present  features  of  great  theo- 
retical interest  and  of  some  technical  importance. 
We  have  already  mentioned  that  the  latter  are  much 
less  affected  by  electrolytes  in  low  concentrations 
than  are  the  suspensoids,  and  the  mixture  acquires 
this  characteristic  ;  the  suspensoid  sol  also  becomes 
less  sensitive  or,  as  the  effect  is  usually  described, 
it  is  "  protected  "  by  the  emulsoid.  The  phenomenon 
had  already  been  observed  by  Faraday,  who  noticed 
that  the  addition  of  "a  little  jelly  "  (presumably 
gelatin)  rendered  his  gold  sols  much  more  stable. 
It  can  be  very  easily  demonstrated  by  adding  to  a 
gold  sol  a  small  quantity  of,  say,  gelatin  or  gum- 
arabic  sol,  and  then  an  amount  of  electrolyte  known 
to  produce  an  immediate  colour  change  in  the  original 
sol,  when  the  protected  sol  will  be  found  to  be 
unaltered.  The  phenomenon  has  been  studied 
quantitatively  chiefly  by  Zsigmondy,  who  defines  as 
the  "  gold  number  "  of  an  emulsoid  the  number  of 
milligrammes  which,  when  added  to  10  c.c.  of  a 
standard  gold  sol,  is  just  insufficient  to  prevent  a 
colour  change  on  the  addition  of  I  c.c.  of  10  per  cent, 
sodium  chloride  solution.  The  reciprocal  of  the  gold 
number  is  therefore  a  measure  of  the  "  protective 
effect."  A  number  of  values  determined  by  Zsig- 
mondy in  this  way  are  given  below  :— 

Reciprocal  of 
Protective  colloid.  Gold  number.  gold  number. 

Gelatin       .  .          .  .  0-005  —  °'01  20°  ~~  I0° 

Casein        .  .          .  .  o-oi  TOO 

Gum-arabic  .  .  0-15  —  0-5  6-7  —  2-0 

Gum-tragacanth  2-0  0-5 

Dextrin      . .          . .  6  —  20  0-17  —  0-05 


64  PROTECTED   SOLS. 

As  the  gold  numbers  represent  milligrammes  in 
ii  c.c.,  or  grammes  in  n  litres,  it  is  evident  that  the 
concentrations  of  even  inferior  protecting  agents 
like  the  gums  are  very  low,  while  those  of  gelatin 
are  exiguous.  It  must  be  added,  however,  that  the 
protective  effect  is  markedly  specific  with  reference 
to  the  sol ;  if  another  sol  is  substituted  for  gold  sol, 
the  concentration  of  any  given  colloid,  and  even  the 
order  in  which  the  gold  numbers  increase,  may  vary 
considerably. 

The  protective  effect  may  also  manifest  itself  in 
another  way  by  preventing  the  formation  of  a 
precipitate  with  particles  exceeding  colloidal  size  as 
the  result  of  a  reaction  which,  in  the  absence  of  the 
protective  agent,  produces  a  coarse  precipitate. 
Sols  of  a  great  variety  of  substances  may  be  obtained 
in  this  way,  by  carrying  out  the  necessary  reactions 
in  the  presence  of  gelatin  or  gum-arabic,  and  the 
concentration  of.  disperse  phase  can  be  increased 
much  beyond  that  obtainable  without  protection. 
In  many  cases  the  sols  thus  made  possess  a  further 
characteristic  of  the  protective  agent,  viz.,  they  may 
be  dried  and  the  residue  redisperses  in  water  or  other 
dispersion  medium.  One  group  of  compounds, 
which  shows  this  effect  to  a  marked  degree,  has 
acquired  great  prominence  in  recent  years.  These 
are  certain  products  of  the  hydrolysis  of  albumin  by 
alkali,  first  described  by  Paal  and  his  collaborators 
as  "  protalbic  "  and  "  lysalbic  "  acids,  and  used  by 
them  in  the  preparation  of  a  very  large  number  of 
metal  and  other  sols. 

Amberger  (1912)  has  shown  that  wool  fat  may  act 
as  a  protective  agent  in  organic  liquids,  in  which  it 
is  soluble,  and  has  prepared  various  "  organosols  " 
of  the  noble  metals  by  its  aid.  The  preparation  of 
silver  sol  is  a  good  instance  of  the  method.  A 
concentrated  solution  of  silver  nitrate  is  thoroughly 
incorporated  with  wool  fat  (which,  as  is  well  known, 


THEORY   OF  PROTECTION.  65 

absorbs  a  large  volume  of  water)  and  then  an 
equivalent  amount  of  caustic  soda  solution.  The 
silver  oxide  formed  is,  under  the  influence  of  light, 
reduced  by  some  of  the  constituents  of  the  fat  ;  the 
whole  mass  is  then  dissolved  in  chloroform  and 
shaken  out  with  calcium  chloride  to  remove  water 
and  reaction  products.  The  chloroform  sol  on 
evaporation  leaves  a  salve-like  residue,  which 
disperses  in  any  solvent  for  wool  fat  to  form  a  dark 
brown  and  extremely  stable  silver  sol. 

The  mechanism  of  protective  action  is  by  no  means 
clear.  The  theory  suggested  by  Bechhold,  that  the 
suspensoid  particles  are  coated  by  the  protective 
colloid,  is  difficult  to  reconcile  with  the  ratios  of  the 
numbers  and  volumes  of  particles  in  highly  disperse 
systems,  though  it  may  quite  possibly  hold  for 
coarser  systems.  That  particles  of  one  colloid 
aggregate  with  a  number  of  particles  of  the  other  is 
maintained  by  Zsigmondy,  as  the  outcome  of  ultra- 
microscopic  observation,  but  this  does  not  explain 
how  the  action  of  electrolytes  on  the  complexes  is 
prevented.  The  matter  is  still  further  complicated 
by  some  observations  made  by  Wo.  Ostwald  (Roll. 
Beih.  10,  179,  1919)  on  Congo-Rubin,  the  red  sodium 
salt  of  a  blue  acid.  Solutions  of  the  dye  show 
typically  suspensoid  character  and  undergo  a  colour 
change  very  similar  to  that  of  red  gold  sols  on  the 
addition,  not  only  of  acids  which  may  be  held  to 
liberate  the  dye  acid,  but  of  neutral  salts  and  even 
of  barium  hydroxide.  Ostwald  finds  that  Congo- 
Rubin  sols  can  be  protected  after  the  addition  of  the 
electrolyte — i.e.,  when  the  protective  colloid  is  added 
to  a  sol  which  has  been  turned  purple  or  blue  by 
electrolyte,  the  colour  returns  to  red.  It  is  difficult 
to  reconcile  this  result — which  by  the  way  does  not 
appear  to  have  been  duplicated  in  the  case  of  any 
other  sol — with  any  existing  theory  of  protective 
action. 


CHAPTER   IX. 

WE  now  proceed  to  the  description  of  a  few  typical 
hydroxide  sols,  a  class  exhibiting  such  a  variety  of 
behaviour,  especially  towards  electrolytes,  as  to 
justify  their  separate  treatment.  In  all  cases  in 
which  a  coagulum  is  produced  by  electrolytes  this 
does  not,  as  in  the  case  of  typical  suspensoids, 
consist  of  the  material  of  the  disperse  phase  only  but 
is  heavily  hydrated  ;  in  a  number  of  cases  no 
macroscopic  separation  of  disperse  phase  occurs,  but 
the  whole  system  sets  to  a  gel,  indicating  emulsoid 
character.  The  methods  used  in  preparing  them 
are,  as  in  the  case  of  suspensoids,  either  condensation 
or  dispersion  methods,  and  among  the  latter  peptiza- 
tion  by  suitable  agents  plays  a  considerable  part. 

Sols  of  aluminium,  ferric  and  chromic  hydroxide 
were  prepared  by  Graham  by  the  same  method  ; 
the  freshly  precipitated  hydroxide  was  dissolved 
in  a  solution  of  the  chloride  and  dialysed.  An 
alternative  procedure  in  the  case  of  ferric  hydroxide 
consists  in  adding  a  solution  of  ammonia  or  ammo- 
nium carbonate  to  ferric  chloride  solution  as  long 
as  the  precipitate  redissolves,  and  then  dialyzing. 
The  sols  are  perfectly  clear  liquids  and  are  very 
sensitive  to  electrolytes,  more  particularly  the 
A1(OH)3  sol. 

The  latter  can  also  be  made  much  more  expedi- 
tiously  by  another  peptization  method  due  to 
A.  Muller.  Aluminium  hydroxide  is  precipitated 
from  hot  dilute  aluminium  chloride  or  sulphate 
solution  by  ammonia,  washed  rapidly,  and  then 
suspended  in  water.  Dilute  hydrochloric  acid  (N/20) 


STANNIC  ACID   SOL.  67 

is  now  added  in  lots  of  i  or  2  c.c.  at  a  time,  and  the 
mixture  heated  to  boiling  after  every  addition, 
evaporated  water  being  replaced.  The  precipitate 

¥radually  disperses,   and  an  opalescent  sol  results, 
horium   hydroxide   sol   may   be   prepared   by   an 
exactly  analogous  manner. 

The  three  sols  mentioned  are  positively  charged 
and  it  is  therefore  the  nature,  and  more  particularly 
the  valency,  of  the  anion  which  determines  the 
concentration  required  to  produce  coagulation. 

A  negatively  charged  hydroxide  sol  with  interesting 
properties  is  that  of  stannic  acid,  which  had  also  been 
prepared  and  studied  by  Graham  ;  more  recently  it 
has  received  a  good  deal  of  attention  from  Zsigmondy 
and  his  school.  The  simplest  way  of  obtaining  it  is 
as  follows  :  a  solution  of  sodium  stannate  is  decom- 
posed by  a  solution  of  sodium  hydrogen  carbonate, 
the  precipitate  well  washed,  suspended  in  water,  and 
peptized  with  2  or  3  c.c.  of  ammonia  solution.  The 
precipitate  gradually  disperses,  the  liquid  becoming 
first  opalescent  and  finally  colourless  and  clear  as 
water.  Very  small  amounts  of  almost  any  electrolyte 
produce  a  copious  flocculent  precipitate.  Notwith- 
standing this  high  sensitiveness  the  sol  shows  one  of 
the  properties  of  emulsoid  sols  :  it  exerts  a  marked 
protective  action  on  suspensoid  sols.  If  a  mixture 
of  stannic  acid  sol  and  red  gold  sol  is  coagulated  by 
an  electrolyte,  a  red  precipitate  results  :  in  other 
words,  the  stannic  acid  has  prevented  the  coagulation 
of  the  gold  while  carrying  it  down  with  it.  It  has 
been  shown  by  Zsigmondy  that  this  precipitate  is 
identical  with  the  purple  of  Cassius,  i.e.,  the  precipi- 
tate obtained  by  reducing  gold  chloride  with  stannous 
chloride.  The  latter,  as  well  as  the  coagulum  from 
stannic  acid  and  gold  sol,  may  be  peptized  by 
ammonia.  Analogous  precipitates  may  be  obtained 
from  mixtures  of  stannic  acid  sol  with  silver,  platinum 
and,  as  Ostwald  has  shown,  Congo-Rubin  sol. 


68  HYDROXIDE   SOLS. 

A  number  of  hydroxide  sols  of  the  tri-  and  quadri- 
valent metals  may  be  prepared  through  the  hydrolysis 
of  their  nitrates,  acetates  or  chlorides,  a  procedure 
which  falls  under  the  head  of  condensation  methods. 
Aluminium  hydroxide  sol  may  be  obtained,  as  a 
slightly  opalescent  liquid,  by  prolonged  boiling  of 
a  dilute  solution  of  aluminium  acetate.  Another 
instance  is  Krecke's  method  of  preparing  ferric 
hydroxide  sol :  if  a  few  c.c.  of  30  per  cent,  solution 
of  ferric  chloride  are  added  to  about  500  c.c.  of 
boiling  water,  the  mixture  turns  reddish  brown  and 
perfectly  clear  in  transmitted  as  well  as  reflected 
light.  The  HC1  formed  by  the  hydrolysis  may  be 
removed  by  dialysing  while  still  hot. 

In  the  case  of  some  nitrates  hydrolysis  is  sufficient 
at  ordina'ry  temperature,  and  it  is  only  necessary  to 
dialyse  their  solutions  against  water  to  obtain  the 
hydroxide  sols.  Probably  the  most  interesting  of 
these  is  the  sol  of  eerie  hydroxide,  first  prepared  by 
A.  Muller  by  dialysing  an  n  per  cent,  solution  of 
eerie  ammonium  nitrate  against  water  for  four  days. 
The  sol  is  a  clear,  yellow  liquid,  which  on  addition  of 
electrolytes  sets  to  a  clear  gel.  The  sensitiveness  to 
the  latter  is  remarkable  ;  a  concentration  of  about 
20  millimoles  of  sodium  chloride  is  sufficient  to 
produce  gel  formation  within  a  few  minutes  in  a 
sol  dialysed  to  a  sufficient  extent.  An  interesting 
property  of  the  sol  is  that,  either  by  ageing  for  several 
months,  or  by  heating  to  100°  C.  for  only  30  minutes, 
it  loses  the  power  of  forming  a  gel,  but  gives  a  preci- 
pitate on  the  addition  of  electrolytes.  This  is  no 
doubt  due  to  gradual  dehydration  of  the  disperse 
phase,  a  change  which  can  be  traced  by  viscosity 
measurements.  The  sol  has  been  studied  in  great 
detail  by  Pauli  and  Fernau  (Koll.-Zeitschr.,  20,  20, 
1917). 

As  is  evident  from  this  short  survey,  the  metallic 
hydroxide  sols  exhibit  a  considerable  variety  of 


COAGULATION   OF  HYDROXIDE   SOLS.    69 

behaviour.  Those  described  are  positively  charged 
with  the  exception  of  stannic  acid,  and  are,  generally 
speaking,  very  sensitive  to  electrolytes.  As  far  as 
the  effect  of  valency  has  been  studied — chiefly  for 
ferric  hydroxide,  and  to  a  smaller  extent  for 
aluminium  hydroxide — it  agrees  with  that  found  to 
hold  for  negative  suspensoid  sols  ;  the  anion  is  the 
ion  which  determines  coagulation  (or  gel  formation), 
and  bivalent  anions  act  in  markedly  lower  concen- 
tration than  univalent  ones.  Ions  of  higher  valency, 
like  Fe(CN)6"",  precipitate  in  extraordinarily  low 
concentrations  and  have  not  been  studied  quantita- 
tively. While  the  electrical  properties  are  thus  fairly 
uniform,  is  is  obvious  that  a  further  factor,  viz., 
hydration  of  the  disperse  phase,  plays  an  important 
part,  as  it  does  in  the  systems  of  undoubted  emulsoid 
character. 


CHAPTER   X. 

WE  have  so  far  considered  typical  suspensoids,  in 
which  the  disperse  phase  consists  of  solid,  or  more 
correctly  rigid,  particles,  and  a  few  metallic  hydroxide 
sols,  in  which  the  disperse  phase  is  undoubtedly 
heavily '  hydrated,  so  that  they  form  a  gradual 
transition  to  the  emulsoid  systems.  To  gain  a 
deeper  insight  into  the  properties,  more  especially 
the  physical  ones,  of  systems  of  two  liquid  phases,  it 
is  necessary  to  consider  dispersions,  both  phases  of 
which  are  known  to  be  liquid  in  bulk  and  under 
ordinary  conditions  of  temperature,  etc.  Such 
dispersions  of  one  liquid  in  another,  in  which  it  is 
practically  insoluble,  are  kno\vn  as  "  emulsions," 
and  play  an  important  part  in  nature  and  in  the 
arts.  The  best  kno\vn  natural  emulsion  is  milk, 
which  contains  fat  globules  dispersed  in  a  solution 
of  caseinogen,  albumin,  sugar  and  various  salts.  As 
regards  artificial  emulsions,  some  of  them  are  unwel- 
come by-products  of  various  industrial  processes, 
such  as  the  condense  water  from  reciprocating  steam 
engines,  which  contains  a  portion  of  the  oil  used  for 
cylinder  lubrication  in  a  state  of  very  fine  and 
persistent  division,  or  wool  \vashings,  in  \vhich  some 
of  the  wool  fat  is  emulsified  by  the  action  of  soaps 
formed  from  certain  constituents  of  the  fat.  A  large 
number  of  emulsions  are  also  prepared  purposely  ; 
mayonnaise  sauce,  an  emulsion  of  oil  in  yolk  of  egg, 
is  a  familiar  and  very  instructive  example,  while 
others  are  well-known  pharmaceutical  preparations, 
certain  "  solid  "  lubricating  compounds,  etc. 

The  simplest  systems  of  this  class  are  the  pure 


PURE   OIL-WATER  EMULSIONS.         71 

"  oil-water  "  emulsions,  in  which,  as  the  name  implies, 
the  continuous  phase  is  water  not  containing  any 
solute.  The  type  of  these  is  engine  condense  water, 
with  about  one  part -of  oil  in  10,000  of  water  ;  very 
similar  emulsions  may  be  obtained  by  pouring  a 
dilute  solution  of  an  oil  in  alcohol  or  acetone  into  a 
large  volume  of  water,  when  the  oil  separates  in  the 
form  of  minute  globules,  generally  less  than  I  p, 
diameter.  In  this  way  it  is^possible  to  obtain  stable 
emulsions  containing  up  to  1/1,000  part  of  oil. 

This  great  dilution  is  quite  in  keeping  with  what 
we  already  know  about  suspensoids,  and  investiga- 
tion of  such  emulsions,  which  has  been  carried  out 
during  the  last  ten  or  twelve  years  by  Donnan, 
Lewis, .  Ellis  and  Goodwin,  and  the  author,  shows 
that  they  do  not  differ  very  materially  from  systems 
with  solid  particles.  The  oil  globules  show  active 
Brownian  movement,  and  are  coagulated  by  electro- 
lytes, acid  being  effective  in  very  much  lower  con- 
centrations than  salts  of  univalent,  and  even  some 
bivalent  cations,  the  charge  on  the  oil  globules  being 
negative.  They  further  resemble  solid  particles, 
inasmuch  as  they  can  be  retained  by  ultra-filtration, 
as  was  first  shown  by  the  author.  This  means  that 
the  globules,  although  liquid,  require  a  considerable 
pressure  to  be  deformed  sufficiently  to  enter  the 
capillaries  of  the  septum.  Mathematical  investiga- 
tion by  the  author,  which  is  in  very  good  agreement 
with  experimental  results,  has  shown  that  this 
"  quasi-rigidity  "  of  the  particles  is  due,  and  simply 
proportional,  to  their  interfacial  tension  against  the 
dispersion  medium,  which  resists  deformation  and 
the  consequent  increase  of  surface.  Oil  globules 
having  a  diameter  of  about  0-8  //,  and  an  interfacial 
tension  (against  water)  of  about  40  dynes/cm,  require 
a  pressure  of  over  5  atmospheres  to  force  them  into 
pores  of  half  that  diameter  filled  with  water.  For 
mercury  globules  of  the  same  size,  but  with  an  inter- 


72  PHASE   RATIO. 

facial  tension  of  about  370  dynes/cm,  the  pressure 
would  be  nine  times  that  for  oil  globules,  so  that  the 
mercury  globules  would  be,  from  this  aspect,  indis- 
tinguishable from  solid  spheres.  If  the  liquid  state 
of  the  disperse  phase  is  to  show  itself,  a  further 
condition  must  be  satisfied,  viz.,  the  interfacial 
tension  of  the  two  phases  must  be  low. 

This  will  become  still  clearer  from  the  following 
considerations.  The  phase  ratio  in  the  case  of  solid 
particles  is  obviously  limited  by  their  shape.  If  we 
take  the  simplest  case,  spherical  particles  of  equal 
diameter,  the  closest  packing  possible  is  reached 
when  each  sphere  touches  12  others,  in  which 
case  the  spheres  occupy  about  74  per  cent,  of  the 
total  volume  and  the  ratio  :  volume  of  disperse 
phase/volume  of  continuous  phase  is  therefore 
about  74/26.  Such  an  arrangement  of  solid  particles, 
however,  no  longer  has  the  properties  of  a  liquid,  but 
will  be  a  mud  or  paste  which  may  retain  its  shape. 
If  the  disperse  phase,  however,  is  liquid  and  easily 
deformed,  there  is  obviously  no  upper  limit  to  the 
phase  ratio  ;  when  the  ratio  76/24  is  reached  the 
globules  will  just  touch,  and  when  it  is  exceeded  they 
will  become  flattened  at  the  12  points  of  contact, 
and  develop  the  faces  of  the  rhombo-dodecahedron. 
As  this  development  of  polyhedral  shape  involves  an 
increase  of  surface,  it  is  opposed  by  the  interfacial 
tension  of  the  phases  and  becomes  possible  only  if 
this  is  low.  In  that  event  there  is  physically  as  well 
as  geometrically  no  upper  limit  to  the  phase  ratio  ; 
as  a  matter  of  fact  emulsions  containing  99  per  cent, 
of  disperse  oil  in  i  per  cent,  of  soap  solution  have  been 
prepared  by  Pickering. 

It  is,  generally  speaking,  quite  impossible  to  make 
emulsions  containing  such  high  percentages  of  dis- 
perse phase  (or,  indeed,  anything  more  than  fractions 
of  i  per  cent.)  in  an  aqueous  dispersion  medium, 
unless  the  latter  is  a  solution  of  certain  substances, 


EFFECT  OF   INTERFACIAL  TENSION.     73 

such  as  a  soap,  various  products  of  the  saponifica- 
tion  of  albumin  or  other  proteins,  or  one  of  the 
saponins.  All  these  solutions  have  one  characteristic 
in  common  :  they  froth  strongly  even  in  great 
dilution.  Frothing,  which  never  occurs  in  pure 
liquids,  is  a  definite  indication  that  the  dissolved 
substance  lowers  the  surface  tension  of  the  solvent, 
and  this  lowering,  which  generally  is  parallel  with 
a  lower  interfacial  tension  against  a  second  liquid, 
is  intimately  connected,  not  only  with  the  possibility 
of  highly-concentrated  emulsions  discussed  above, 
but  with  the  process  of  emulsification.  This  connec- 
tion can  be  easily  demonstrated  with  the  drop 
pipette  as  modified  by  Donnan,  illustrated  in  Fig.  9. 
A  pipette  A  is  provided  with  a  length  of  capillary 
tube  B,  the  object  of  the  latter  being  to  restrict  the 
flow.  The  bend  C  ends  in  a  point  D,  which  is  ground 
off  flat.  The  pipette  is  .filled  with  the  oil  to  be 
examined  and  the  outlet  submerged  in  the  solution 
which  is  to  form  the  continuous  phase.  The  size 
of  the  drop  which  issues  from  D  is  determined  on 
the  one  hand  by  the  difference  in  density  of  the  two 
liquids,  which  tends  to  tear  it  off,  and  on  the  other 
by  the  interfacial  tension  acting  round  the  circum- 
ference of  the  orifice,  which  tends  to  retain  it.  Any 
decrease  in  interfacial  tension  accordingly  shows 
itself  in  reduced  size,  or  increased  number,  of  drops, 
and  the  parallelism  between  it  arid  the  emulsifying 
power  of  the  solution  is  very  marked.  To  give  a 
numerical  instance,  a  pipette  used  by  the  author 
gave  65  drops  of  light  petroleum  in  water,  and  260 
drops  of  the  same  oil  in  I  per  cent,  soap  solution. 
With  such  a  soap  solution  it  is  quite  easy  to  make 
emulsions  containing  80  per  cent,  of  petroleum  as 
disperse  phase. 

We  can  now  form  a  picture  of  the  various  factors 
which  make  possible  the  existence  of  these  high 
percentage  emulsions  and  give  them  their  highly 


74 


DROP   PIPETTE. 


characteristic  properties.     The  oil  globules  are  no 
longer  spherical  but  polyhedral,  the  adjoining  faces 


FIG.  9. — DONNAN'S  DROP  PIPETTE. 

being  separated  by  very  thin  films  of  the  disperse 
phase.  Such  films  would  tear  if  they  had  the  high 
surface  tension  of  water,  and  can  persist  only  if  the 
interfacial  tension  is  very  greatly  lowered  by  the 


INTERFACIAL  LAYERS.  75 

dissolved  substances  mentioned  above.  If  the  films 
are  thick,  i.e.,  if  the  particles  are  widely  separated, 
a  persistent  emulsion  is  possible  even  in  pure  water. 
At  the  same  time,  the  low  interfacial  tension  alone 
does  not  appear  sufficient  to  prevent  the  eventual 
fusion  of  contiguous  globules  of  disperse  phase,  and 
several  investigators,  especially  W.  D.  Bancroft, 
have  arrived  at  the  conclusion  that  the  particles 
must  be  surrounded  by  a  film  having  certain  definite 
properties,  into  which  we  cannot  enter.  Such  a 
film  may  be  the  result  of  adsorption  from  a  solution 
(see  Chapter  XXL),  or  it  may  even  consist  of  a 
finely  divided  solid.  Pickering  showed  that  if 
certain  basic  sulphates  of  heavy  metals  are  shaken 
up  with  water  and  oil,  the  latter  is  dispersed  in  the 
former  as  a  stable  emulsion.  Conversely  Schlaepfer 
(//.  Chem.  Soc.,  113,  522,  1918),  was  able  to  produce 
an  emulsion  of  water  in  oil  by  shaking  the  two  liquids 
with  lamp  black.  The  type  of  emulsion  formed 
evidently  depends  on  whether  the  finely-divided 
solid  is  more  easily  wetted  by  one  phase  than  by 
the  other,  and  the  liquid  which  wets  less  becomes 
the  disperse  phase,  viz.,  oil  with  the  basic  sulphates, 
and  water  with  lamp  black.  The  theory  of  the 
phenomenon  is  treated  at  greater  length  in  the 
companion  volume  on  "  Surface  Tension  and  Surface 
Energy,"  by  R.  S.  Willows  and  the  author. 

If  the  emulsifying  agent  is  destroyed,  for  example, 
by  adding  a  mineral  acid  to  an  emulsion  of  oil  in 
soap  solution,  the  disperse  phase  coalesces  imme- 
diately and  eventually  separates  completely.  This 
segregation  is  of  course  simply  the  result  of  the 
great  increase  in  interfacial  tension  when,  say, 
sodium  chloride  is  formed  from  the  sodium  oleate 
present  on  the  addition  of  hydrochloric  acid.  A 
phenomenon  of  great  interest,  and  less  easy  to 
explain,  was  first  observed  by  G.  A.  Clowes.  An 
emulsion  of  a  glyceride,  e.g.,  olive  oil,  can  easily  be 


76  PHASE   REVERSAL. 

made  by  shaking  it  with  very  dilute  caustic  soda, 
the  small  amount  of  oleate  formed  acting  as  emulsify- 
ing agent  ;  the  oil  becomes  the  disperse  phase.  If 
an  equivalent  amount  of  calcium  chloride  is  added 
to  the  emulsion,  the  phases  are  reversed,  the  oil 
becoming  the  continuous,  and  the  aqueous  solution 
the  disperse  phase.  Other  electrolytes  have  been 
systematically  studied  by  Bhatnagar  (Gen.  Disc., 
Faraday  Soc.,  October,  1920),  but  the  theory  of  this 
interesting  phenomenon  cannot  yet  be  said  to  be 
clear. 

Generally  speaking,  we  find  that  emulsions  group 
themselves,  as  far  as  the  conditions  for  stability  are 
concerned,  into  two  classes.  In  the  pure  oil-water 
emulsions,  as  in  the  suspensoids,  the  stabilizing 
factor  is  undoubtedly  the  electric  charge.  In  the 
concentrated  emulsions  made  with  an  emulsifying 
agent  the  electric  conditions  have  hardly  been 
investigated  and  can  only  play  a  secondary  part  : 
the  stability  depends  on  other  physical  properties  of 
the  continuous  phase,  and  probably  on  the  nature 
of  some  film  formed  at  the  boundary  of  the  two 
phases. 

The  viscosity  of  emulsions  has  received  very  little 
quantitative  study,  but  it  is  known  to  increase  very 
rapidly  with  the  percentage  of  disperse  phase.  A 
marked  difference  between  a  suspension  of  rigid 
particles,  and  an  emulsion  can,  in  the  nature  of 
things,  be  expected  to  show  itself  only  when  the 
globules  of  disperse  phase  approach  contact,  i.e., 
when  the  disperse  phase  amounts  to  more  than 
70  per  cent,  of  the  total  volume. 

The  question  now  arises  how  far  the  considera- 
tions on  the  properties  of  systems  of  two — known — 
liquid  phases  may  be  transferred  to,  or  help  to 
elucidate,  the  properties  of  emulsoid  sols,  in  which 
the  liquid  state  of  the  dispersed  phase  is  merely  an 
inference.  It  is  fairly  obvious  that  another  factor 


HYDRATED   EMULSOIDS.  77 

must  be  presumed,  more  especially  from  considera- 
tions of  viscosity.  We  have  already  seen  that  small 
concentrations  of  emulsoid  colloids,  such  as  gelatin, 
rubber  or  nitre-cellulose,  may  produce  sols  of  very 
high  viscosity  ;  on  the  other  hand,  we  see  in  the 
case  of  emulsions,  that  a  high  viscosity  is  caused 
only  by  a  large  volume  percentage  of  disperse  phase. 
We  are  therefore  forced  to  the  conclusion  that  a 
small  amount  of  dry  emulsoid  may  produce  a  large 
volume  of  disperse  phase  in  the  sol,  by  hydration  or 
solvation,  that  is  by  associating  with  itself  a  large 
portion  of  the  solvent.  The  evidence  for  this 
condition  is  varied  and  conclusive,  and  we  shall 
further  see  that  the  state  of  hydration  is  also 
intimately  connected  with  the  electric  condition  of 
the  disperse  phase  in  a  number  of  sols.  Individual 
sols  show — compared  with  the  striking  uniformity 
of  suspensoid  sols — a  bewildering  variety  of 
behaviour,  and  must  therefore  be  studied  separately. 
As  a  first  representative  of  the  class  we  will  consider 
the  principal  inorganic  emulsoid  sol,  that  of  silicic 
acid. 


CHAPTER   XI. 

"  SOLUBLE  "  silicic  acid  was  known  already  to 
Berzelius,  who  obtained  it  by  treating  silicon  sulphide 
with  water.  Hydrogen  sulphide  is  liberated  and 
the  silicic  acid  formed  at  the  same  time  remains  in 
solution.  Berzelius  also  noticed  that  more  concen- 
trated solutions  became  gelatinous  after  a  time. 
These  observations  were  repeated  by  Fremy,  who 
found  that  the  addition  of  alkali  salts  caused  the 
solution  to  gelatinize. 

The  silicic  acid  sol  was  studied  extensively  by 
Graham.  He  prepared  it  by  pouring  a  solution  of 
sodium  silicate  into  an  excess  of  dilute  hydrochloric 
acid,  and  dialyzing  the  mixture  to  remove  the  sodium 
chloride  formed  and  the  excess  of  acid.  During 
the  first  stage  of  dialysis  silicic  acid  passes  freely 
into  the  outside  water  :  the  portion  which  thus 
escapes  may  amount  to  10  per  cent,  of  the  total. 
The  liquid  finally  left  in  the  dialyzer  is  perfectly 
clear  and  colourless,  and  has  a  viscosity  not  much 
in  excess  of  that  of  water.  On  keeping,  or  on 
addition  of  certain  electrolytes,  the  sol  becomes 
opalescent,  the  viscosity  increases  rapidly  and  the 
sol  is  finally  transformed  into  a  translucent  gel. 
The  transformation  is  irreversible,  i.e.,  the  gel  cannot 
be  transformed  back  into  sol  by  treatment  with 
water  alone. 

The  electrical  properties  of  the  sol  and  the  effect 
of  electrolytes  are  rather  obscure,  notwithstanding 
a  considerable  number  of  investigations  devoted 
to  them.  Graham  already  observed  that  certain 
electrolytes  produced  very  rapid  gel  formation  : 


INORGANIC   EMULSOID   SOLS.  79 

ammonia,  phosphates  and  carbonates,  and  even  CO^ 
passed  through  the  sol,  act  in  this  fashion.  Generally 
speaking,  -a  small  amount  of  acid  increases  the 
stability  ;  this  is  of  particular  interest,  since  Billiter 
has  shown  by  cataphoresis  that  the  sol  is  electrically 
neutral  with  this  concentration  of  H  ion,  while  it  is 
positively  charged  with  higher  H '  concentration,  and 
negative  when  containing  OH'.  The  sol  accordingly 
appears  to  have  a  maximum  of  stability  at  the  iso- 
electric  point,  a  state  of  things  which  we  shall 
encounter  in  several  emulsoid  sols,  but  one  which  is 
in  sharp  contrast  to  the  behaviour  of  suspensoids. 
The  effect  of  neutral  salts  has  been  studied  by 
Pappada  among  others  ;  here  again  there  is  a  marked 
difference  between  silicic  acid  sol  and  suspensoid 
sols,  inasmuch  as  very  much  higher  concentrations 
are  necessary  to  accelerate  gel  formation  than  those 
which  produce  coagulation  in  suspensoids.  This  also 
is  a  phenomenon  which  occurs  with  a  number  of 
emulsoid  sols.  Furthermore,  a  marked  specific  effect, 
especially  of  the  anion,  shows  itself,  which  is  not  con- 
nected with  its  valency.  This  effect,  which  is  of  very 
great  importance,  we  shall  discuss  in  detail  when 
describing  the  albumin  sol,  in  connection  with  which 
it  was  first  observed  and  has  been  most  thoroughly 
studied. 

The  eerie  hydroxide  already  mentioned  (p.  68), 
while  showing  distinctly  the  character  of  a  positively 
charged  sol,  has  marked  emulsoid  properties  when 
fresh.  Like  the  silicic  acid  sol  it  sets  to  a  gel  on 
addition  of  electrolytes,  but  this  transformation  is 
not  necessarily  irreversible  ;  a  fresh  gel  may  be 
peptized  again  by  contact  with  the  sol,  or  by  the 
addition  of  an  extremely  small  amount  of  nitric 
acid.  We  have  already  seen  that  the  sol  may  lose 
the  emulsoid  character  and  eventually  give  a  pre- 
cipitate on  addition  of  electrolytes  ;  we  shall  again 
find  parallels  to  this  change  in  organic  emulsoids. 


8o  INORGANIC  EMULSOID   SOLS. 

An  emulsoid  sol  showing  marked  indifference  to 
electrolytes  is  that  of  tungstic  acid,  obtained  accord- 
ing to  Graham  by  adding  a  slight  excess  of  dilute 
hydrochloric  acid  to  a  solution  of  sodium  tungstate 
and  dialysing  the  mixture.  A  little  hydrochloric  acid 
must  be  added  at  intervals  during  dialysis ;  a 
perfectly  clear  sol  of  tungstic  acid  finally  remains, 
which  neither  precipitates  nor  sets  to  a  jelly  on  the 
addition  of  acids  or  neutral  salts.  It  can  be  evapo- 
rated to  dryness,  the  residue  forming  transparent 
scales  which  resemble  gelatin  and,  like  the  latter, 
adhere  so  strongly  to  the  glaze  of  the  basin  as  to  tear 
fragments  out  of  it  during  the  progress  of  drying. 
The  dry  acid  forms  a  gummy  mass  and  finally  a  sol 
with  water.  The  resemblance  to  organic  emulsoids 
like  gum-arabic  is  thus  very  striking. 

The  three  inorganic  emulsoid  sols  described  show 
the  variety  of  behaviour  which  we  shall  find  again, 
to  an  even  more  striking  degree,  among  the  important 
organic  representatives  of  the  class.  Ceric  hydroxide 
is  distinctly  a  positive  sol  ;  silicic  acid  is  also  affected 
by  electrolytes,  but  neither  the  sign  of  the  charge 
nor  the  effects  of  valency  are  as  unambiguous  as 
with  the  suspensoids,  while  tungstic  acid  is  apparently 
indifferent  to  electrolytes.  It  must  be  added  that  the 
last  -  named  sol  has  not  received  attention  since 
Graham's  time. 

Silicic  acid  sols  and  gels  are  supposed  to  have 
existed  in  considerable  quantities  at  some  geological 
periods,  and  various  minerals,  like  agate  and  opal, 
probably  owe  their  origin  to  such  gels  ;  some 
varieties  of  the  latter  contain  fairly  large  amounts 
of  water.  Even  moist  deposits  of  silicic  acid  gel  have 
been  found  in  quite  recent  times  in  the  course  of 
tunnelling  and  mining  operations. 


CHAPTER  XII. 

THE  organic  emulsoids  are  very  numerous  and 
extremely  important,  including,  as  they  do,  most  of 
the  proteins,  such  as  albumin,  casein,  glutin  (gelatin), 
haemoglobin,  etc.,  a  number  of  carbohydrates,  as 
starch,  agar,  the  gums,  cellulose  and  its  various 
esters,  the  soaps,  etc.  It  is  impossible  within  the 
limits  of  a  short  work  to  do  more  than  select  a  few 
typical  examples  and  to  develop  a  number  of  general 
points  of  view  to  which  their  extremely  varied 
behaviour  becomes  referable. 

Two  of  these  may  usefully  be  considered  together, 
viz.,  gelatin  and  agar,  as  illustrating  that  a  marked 
similarity  in  eolloidal  characteristics  may  go  together 
with  a  profound  difference  in  chemical  constitution. 
The  former  is  a  mixture  of  proteins,  the  principal 
one  of  which  is  glutin,  while  agar  is  a  mixture  of 
carbohydrates,  the  most  important  one  being 
^-galactan.  The  two  substances  occur  in  commerce 
as  air-dry  gels  ;  when  immersed-  in  cold  water  they 
imbibe  large  volumes  and  swell  until  an  equilibrium 
is  attained.  On  heating,  the  swollen  gels  disperse  to 
form  sols,  the  temperature  necessary  to  effect  this 
being  25°  to  35°  C.  for  gelatin,  and  about  boiling 
point  for  agar.  On  cooling  the  sols  set  to  translucent 
gels,  the  setting  point  for  gelatin  being  a  few  degrees 
lower  than  the  "  melting  point,"  while  agar  sol  can 
be  cooled  to  about  35°  before  setting.  The  process 
is  completely  reversible,  but  the  gels  have  to  be  heated 
to  the  original  dispersion  temperatures  to  be  trans- 
formed into  sols  again.  The  agar  sol-gel  transforma- 
tion is  thus  a  particularly  striking  example  of  the 


82  HYSTERESIS   OF   SETTING. 

phenomenon  of  hysteresis — which  we  shall  encounter 
constantly  in  connection  with  various  properties  of 
emulsoid  sols — as  the  sol,  once  formed,  does  not 
gelatinize  until  cooled  to  about  35°  C.,  whereas  the 
gel  is  not  retransformed  into  sol  unless  raised  to  a 
temperature  nearly  70°  higher.  A  similar,  though 
much  slighter,  hysteresis  is  observed  with  gelatin, 
the  range  being  from  5°  to  7°.  As  in  the  case  of 
silicic  acid  the  transformation  of  sol  into  gel  is  quite 
continuous  as  far  as  it  can  be  observed.  With  falling 
temperature  the  viscosity  rises  quite  continuously, 
but  eventually  reaches  a  point  where  it  can  no  longer 
be  measured  by  the  usual  methods,  and  a  discon- 
tinuity is  quite  possible  at  or  beyond  this  point. 

Agar  sol  and  gel  have  not  received  much  study, 
and  we  shall  therefore  have  to  confine  ourselves  to  a 
discussion  of  the  properties  of  gelatin.  A  certain 
minimum  concentration  is  necessary  if  a  gel  is  to 
form  at  a  given  temperature  ;  this  varies  considerably 
with  the  brand  of  material,  but  it  may  roughly  be 
stated  that  with  less  than  about  0-25  per  cent,  a 
gelatin  sol  will  not  set  even  at  o°  C.  At  room 
temperature — 15°  C. — a  concentration  of  something- 
like  i  per  cent,  is  necessary  ;  with  increasing  gelatin 
content  the  "  melting  "  and  "  setting  "  points  both 
rise.  The  following  data,  determined  by  Pauli,  give 
an  idea  of  this  connection  and  of  the  hysteresis 
range :- 

Gelatin  concentration 

in  per  cent.  5  10  15 

Setting  temperature    .  .     17-8°  21°  29-5° 

Melt'ng  temperature    .  .      26-1°  29-6°  29-4° 

The  two  temperatures  are  not  sharply  denned,  and 
must  be  determined  by  strict  adherence  to  some 
conventional  method. 

A  fact  of  great  importance  is  that  all  the  properties 
of  a  gelatin  sol  depend  not  only  on  concentration  and 


EFFECT  OF  AGEING.  83 

temperature,  as  do  those  of  a  true  solution,  but  to  a 
marked  degree  on  the  previous  treatment  and  history 
of  the  sol.  The  property  most  conveniently  and 
accurately  measured  is  the  viscosity,  although  in  the 
present  state  of  our  theoretical  knowledge  it  is 
unfortunately  not  possible  to  say  what  physical 
change  in  the  sol  corresponds  to  a  change  in  the 
viscosity  coefficient.  We  shall  return  to  this  subject 
when  a  number  of  emulsoid  sols  have  been  described, 
but  in  the  meantime  it  may  be  said,  for  instance, 
that  the  viscosity  of  a  gelatin  sol  depends  not  only 
on  the  concentration  and  temperature,  but  also  on 
the  age  of  the  sol.  If  a  gelatin  sol  of  such  low  concen- 
tration that  it  does  not  set  at  ordinary  temperature 
is  kept  with  aseptic  or  antiseptic  precautions,  the 
viscosity  rises  steadily  during  the  first  few  days. 
In  an  example  measured  by  the  author  the  viscosity 
of  a  0-5  per  cent,  gelatin  sol  was  969  (in  arbitrary 
units)  when  the  sol  was  24  hours  old,  and  2,657  when 
it  was  48  hours  old,  i.e.,  over  2-5  as  high.  The  vis- 
cosity of  a  sol  even  varies — all  other  factors  being 
equal — according  to  the  time  which  the  dry  gelatin 
has  been  allowed  to  swell  in  water  before  dispersion, 
and  generally  with  every  alteration  in  the  method  of 
preparation. 

The  action  of  electrolytes  on  gelatin  sols  is  exces- 
sively complicated,  and  two  effects  have  to  be 
sharply  distinguished  :  coagulation  or  precipitation, 
and  alterations  in  the  setting  point.  As  regards  the 
former,  very  high  concentrations  are  required  to 
produce  a  coagulum  or  even  incipient  turbidity  : 
several  moles  per  litre  of  KC1  and  about  0-38  moles 
of  Na2SO4.  The  sulphates  precipitate  in  the  lowest 
concentration,  and  a  sufficient  addition  of  sodium 
or  ammonium  sulphate  to  a  gelatin  sol  produces  a 
tough  and  stringy  coagulum,  which,  as  mentioned 
above  (p.  30),  on  standing  at  30°  gradually  forms  a 
liquid  layer.  This  form  of  coagulation  is  evidently 

6—2 


84          EFFECT  OF  NEUTRAL   SALTS. 

different  from  that  familiar  to  us  with  suspensoid 
sols,  and  depends  on  a  withdrawal  of  water  from  the 
disperse  phase.  It  is  in  accordance  with  this  view 
that  gelatin  can  be  precipitated  reversibly  from  its 
sol  by  non-electrolytes  which  withdraw  water,  e.g., 
alcohol.  This  type  of  precipitation  is  therefore 
necessarily  connected  with  the  degree  of  hydration 
which  in  its  turn  depends  on  the  electric  charge  of 
the  disperse  phase  and  becomes  a  minimum  at  the 
iso-electric  point.  Gelatin  is  iso-electric  at  a  definite 
H  concentration,  which  varies  with  different  brands  ; 
Pauli  ("  Colloid  Chemistry  of  the  Proteins,"  p.  38) 
gives  it  as  1-8  X  io~5  for  a  i  per  cent,  sol  of  the  gelatin 
examined  by  him.  At  this  point  the  sol  is  most 
easily  precipitated  by  alcohol  and  the  viscosity— 
which  undoubtedly  increases  with  hydration  of  the 
disperse  phase,  though  the  function  connecting  the 
two  is  not  known — shows  a  decided  minimum. 

The  effect  of  neutral  salts  on  the  setting  tempera- 
ture and  setting  velocity  has  been  studied  by  several 
observers.  A  marked  influence  of  the  anion  is 
unmistakable  :  sulphates  and  acetates  raise  the 
setting  point,  compared  with  that  of  the  aqueous  sol, 
while  nitrates,  iodides  and  thiocyanates  lower  it. 
Incidentally  it  may  be  mentioned  that  these  anions 
have  a  very  similar  effect  on  agar  sols,  and,  as  far 
as  a  parallel  is  possible,  on  silicic  acid  sol,  so  that  the 
effect  is  certainly  not  chemical  or  in  any  way  specific 
to  gelatin.  Ten  per  cent,  sols  of  gelatin  about  half- 
saturated  with  potassium  iodide  or  thiocyanate  do 
not  set  at  all  at  10°  C.,  and  what  comes  to  the  same 
thing,  gelatin  disperses  in  these  salt  solutions  even 
in  the  cold.  A  parallel  effect  on  the  viscosity  has 
also  been  observed,  inasmuch  as  the  salts  which 
raise  the  setting  point  increase  the  viscosity  at  a 
given  temperature,  and  vice  versa.  Certain  non- 
electrolytes  also  raise  the  setting  point,  etc.,  among 
which  substances  containing  a  large  number  of 


OPTICAL   PROPERTIES.  85 

hydroxyl  groups  are  conspicuous,  e.g.,-  several  sugars. 
The  effect  of  the  cations  of  neutral  salts  is  less  well 
marked,  but  a  certain  order  is  also  observable.  We 
shall  find  parallel  effects  on  the  physical  properties 
of  gelatin  gels,  and  on  albumin  sols,  where  the 
anion  series  will  receive  detailed  treatment. 

The  optical  properties  of  gelatin  sols  have  received 
some  attention.  The  refractive  index  has  been 
studied  by  G.  S.  Walpole,  who  finds  it  to  be  a  linear 
function  of  the  concentration  for  a  given  tempera- 
ture ;  since  the  hysteresis  range  of  setting  amounts 
to  several  degrees,  it  is  possible  to  determine  the 
refractive  index  of  sol  and  gel  at  the  same  tempera- 
ture, and  the  two  values. are  found  to  be  the  same. 

Gelatin  sols  are  optically  active  and  show  to  a 
very  striking  degree  the  phenomenon  of  multi- 
rotation,  i.e.,  the  specific  rotation  increases  with  the 
age  of  the  sol,  a  maximum  being  reached  only  after  a 
few  days.  If  the  sol  or  gel  is  warmed,  the  rotation 
falls  again  to  something  like  the  original  value. 

Prolonged  heating  of  a  gelatin  sol  produces  a 
profound  change  which  shows  itself  on  cooling,  when 
the  transformed  sol  no  longer  sets  to  a  jelly.  The 
product  of  transformation  is  known  as  /3-gelatin  or 
gelatose  ;  since  small  amounts  of  it  are  formed  even 
during  short  exposure  to  high  temperatures,  varying 
quantities  of  it  are  always  present  in  gelatin  and 
have  some  effect  on  its  properties. 


CHAPTER   XIII. 

A  BEHAVIOUR  differing  entirely  from  that  of 
gelatin  is  shown  by  the  albumins,  which  are  perhaps 
the  most  important  among  the  organic  emulsoids, 
and  have  received  an  amount  of  study  commensurate 
with  their  importance.  Within  the  limits  of  the 
present  work  it  is  impossible  to  give  more  than  a 
mere  outline  of  the  principal  properties  of  one  type. 
As  such  we  may  take  neutral  albumin  prepared  from 
white  of  egg.  The  latter  contains,  in  addition  to 
albumin,  other  proteins,  viz.,  globulin  and  ovomucoid 
which  are  precipitated  by  the  addition  of  sufficient 
ammonium  sulphate  to  produce  a  half-saturated 
solution.  The  clear  liquid  remaining  after  removal 
of  the  precipitate  contains  the  albumin  :  the 
ammonium  sulphate  is  removed  by  prolonged 
dialysis,  when  a  sol  of  almost  pure  albumin  is  left 
in  the  dialyser.  Some  of  the  earlier  investigations, 
however,  were  carried  out  with  the  whole  white 
of  egg,  i.e.,  a  mixture  of  albumin  and  the  other 
proteins  :  the  procedure  of  Hofmeister  (see  below) 
consisted  in  beating  the  white  to  a  froth,  allowing 
this  to  stand  in  a  tall  cylinder,  and  collecting 
the  clear  liquid  which  separates  from  a  fibrous 
coagulum. 

Egg  albumin  disperses  in  water  at  ordinary 
temperature  to  form  a  slightly  opalescent  sol,  which 
does  not  form  a  gel  either  with  increasing  concentra- 
tion, nor  on  cooling  or  ageing.  On  the  other  hand, 
it  shows  a  specific  phenomenon  :  it  coagulates 
irreversibly  on  heating  to  about  60°  C.  The  heat 
coagulation  has  been  studied  extensively,  especially 


SALTING   OUT.  87 

by  Chick  and  Martin  among  others,  and  consists 
essentially  of  some  chemical  change  followed  by 
coagulation  or  aggregation.  The  second  step  may 
be  prevented  by  suitable  agencies  ;  if  a  well  dialysed 
albumin  sol  containing  a  thiocyanate  is  heated  above 
the  coagulation  temperature  no  change  can  be 
observed,  but  has  nevertheless  taken  place,  as  the 
albumin  is  precipitated  when  the  salt  is  removed  by 
dialysis,  i.e.,  the  thiocyanate  prevents  the  aggregation 
of  the  altered  albumin  particles  but  not  their  trans- 
formation. Other  salts  also  affect  the  temperature 
at  which  heat  coagulation  occurs. 

In  close  connection  with  this  property  of  the 
albumin  sol  is  its  behaviour  to  salts  in  the  cold.  On 
the  addition  of  salts  in  suitable  concentrations  the 
sol  becomes  turbid  and  the  albumin  finally  settles 
out  in  flocculent  masses.  The  coagulum,  however, 
shows  different  characteristics  with  different  salts, 
and  these,  if  for  the  moment  we  consider  only  the 
cation,  divide  themselves  into  three  groups.  The 
salts  of  the  alkalies  and  of  magnesium  produce  coagu- 
lation or  "  salting  out  "  only  in  great  concentrations, 
and  the  process  is  reversible  ;  on  dilution  or  removal 
of  the  salt,  say  by  dialysis,  the  albumin  again 
disperses.  The  salts  of  the  alkaline  earths  salt  out 
in  similar  concentrations,  but  the  precipitate  becomes 
insoluble  on  standing  even  for  a  short  time.  The 
salts  of  the  heavy  metals  finally  salt  out  irreversibly 
even  in  low  concentrations,  but  show  a  further 
peculiarity  :  if  the  concentration  of  salt  is  increased 
beyond  that  necessary  for  precipitation,  the  coagulum 
disperses  again,  and  this  zone  of  redispersion  may  be 
followed,  if  the  salt  concentration  is  increased  still 
further,  by  a  second  zone  of  coagulation.  Thus  zinc 
salts  precipitate  a  10  per  cent,  albumin  sol  in  concen- 
trations from  N/i,ooo  to  N/2  ;  at  this  concentration 
the  coagulum  redissolves  and  remains  so  until  the 
concentration  reaches  4N,  when  a  second  precipita- 


88  THE   HOFMEISTER   SERIES. 

tion  occurs.  The  coagulum  always  contains  the 
metal,  but  in  very  variable  proportions. 

If  we  now  consider,  instead  of  a  variety  of  salts 
with  different  cations,  a  series  having  the  same 
cation — K-  or  Na — we  become  acquainted  with  a 
series  of  the  greatest  importance,  which  was  first 
discovered  by  Hofmeister,  and  is  generally  called 
after  him.  Some  of  his  results  are  given  in  the 
following  table,  which  shows  the  concentration  in 
moles  per  litre  of  the  sodium  salts  of  various  acids 
necessary  to  salt  out  the  same  albumin  at  30°  to 
40°  C.  :— 

Citrate  . .  . .  .  .  0-56 

Tartrate  . .  .  .  .  .  078 

Sulphate  . .  . .  .  .  0-80 

Acetate  . .  .  .  . .  1-69 

Chloride  . .  . .  3-62 

Nitrate  . .  . .  . .  5-42 

Chlorate  ,. .  . .  . .  5-52 

Iodide  .  .  .  .  .  .  \  Do  not  salt  out  in 

Thiocyanate  .  .  .  .  (saturated  solution. 

Hofmeister's  experiments  were  made  with  albumin 
containing  the  other  proteins  of  white  of  egg.  The 
effect  is  roughly  the  same  on  pure  albumin  sols.  A 
further  striking  property  has  been  demonstrated  by 
Pauli ;  the  effect  of  the  series  is  reversed  in  slightly 
acid  sols,  in  which  iodide  and  thiocyanate  have  the 
greatest,  and  citrates  and  tartrates  the  least  action. 

The  preponderating  effect  of  the  anion  on  the 
coagulation — or  other  changes  of  state,  such  as  gel 
formation — in  other  emulsoids  has  been  referred  to 
already.  A  chemical  action  seems  out  of  question 
when  we  bear  in  mind  that,  as  far  as  a  parallel  is 
possible,  the  anions  act  in  a  similar  way  not  on  y  on 
gelatin,  which  is  a  protein,  but  on  substances  as 
dissimilar  as  agar  and  even  silicic  acid.  In  addition 
to  this,  the  anions  arrange  themselves  more  or  less 


ACID-  AND   ALKALI-ALBUMIN.          89 

exactly  in  the  order  found  by  Hofmeister,  if  their 
influence  on  a  number  of  other  phenomena  is  studied, 
e.g.,  on  the  surface  tension  and  compressibility  of 
water,  on  the  solubility  of  other  substances,  on  the 
inversion  of  cane  sugar  and  the  saponification  of 
esters.  With  the  latter  processes  we  even  find  the 
inversion  of  the  series  according  to  the  reaction — 
acid  or  alkaline — of  the  medium. 

Freundlich,  who  has  pointed  out  the  connection 
between  these  various  effects,  calls  them  "  lyotropic," 
and  suggests  that  they  are  brought  about  by  the 
neutral  salt  affecting  the  state  of  molecular  aggrega- 
tion of  the  water,  and  consequently  the  degree  of 
hydration  of  solutes  and  disperse  phases.  The 
explanation  is  the  most  acceptable  one  which  we 
have  at  present,  and  the  generality  of  the  lyotropic 
effect  makes  it  eminently  probable  that  it  is  due  to 
alterations  in  the  properties  of  the  solvent  or  disper- 
sion medium.  We  shall  encounter  it  once  more 
when  discussing  the  properties  of  elastic  gels,  which 
are  profoundly  affected  by  the  anions  of  the  lyotropic 
series. 

The  action  of  acids  and  alkalies  on  albumin,  and 
on  proteins  generally,  is  extremely  complicated,  and 
can  only  be  touched  on  briefly.  These  substances 
are  amphoteric  electrolytes,  i.e.,  they  behave  as 
weak  bases  towards  acids,  and  as  weak  acids  towards 
bases.  Protein  salts  are  thus  formed  with  either, 
which  dissociate  into  protein  cations  and  the  anion 
of  the  acid,  or  protein  anions  and  the  cation  of  the 
base.  With  increasing  acid  or  alkali  concentration 
the  protein  undergoes  gradual  and  irreversible 
chemical  changes,  which  are  more  marked  in  the 
case  of  alkalies.  In  accordance  with  the  above 
statement,  albumin  in  acid  medium  is  positively 
charged  and  travels  to  the  cathode,  while  in  alkaline 
medium  it  is  negative  and  travels  to  the  anode.  The 
electric  neutrality  of  albumin  freed  as  far  as  possible 


90  PROTEIN   SALTS. 

from  electrolytes  by  very  prolonged  dialysis  was 
first  demonstrated  by  Pauli.  He  submitted  the 
purified  sol  to  electrophoresis  in  three  small  beakers 
connected  by  syphons,  the  electrodes  being  immersed 
in  the  two  outside  beakers.  After  the  sol  had  been 
exposed  to  the  electric  field  for  a  time,  the  albumin 
concentration  in  the  beakers  was  determined,  e.g.,  by 
nitrogen  determination  by  the  Kjeldahl  method. 

The  hydration  of  the  disperse  phase,  and  the 
stability  and  viscosity  of  the  albumin  sol  are  inti- 
mately connected  with  the  degree  of  dissociation. 
In  acid-albumin  mixtures  the  stability  and  viscosity 
reach  a  maximum  when  the  ionization  is  at  a 
maximum  ;  both  are  at  a  minimum  at  the  iso-electric 
point. 

As  salts  of  a  weak  acid  or  a  weak  base  the  protein 
salts  also  undergo  hydrolysis.  If  albumin  is  treated 
with  acid  in  low  concentration  for  a  short  time  and 
at  low  temperature,  the  latter  can  be  removed  again 
by  dialysis,  leaving  the  albumin  unaltered  ;  the  acid 
thus  removed  is  no  doubt  liberated  by  hydrolysis,  as 
in  the  dialysis  of  the  nitrates  of  tri-  or  quadrivalent 
metals  (see  p.  68).  It  will  readily  be  realized  how 
complicated  are  the  conditions  in  albumin  sols  con- 
taining acids  or  bases,  and  readers  interested  in  this 
part  of  the  subject  must  be  referred  to  the  large 
literature  on  the  subject,  more  especially  to  Pauli's 
"  Colloid  Chemistry  of  the  Proteins  "  (London, 
J.  and  A.  Churchill). 

Other  proteins  can  receive  only  brief  mention. 
Among  them  are  the  globulins,  already  referred  to  ; 
they  do  not  disperse  in  water,  but  only  in  salt  solutions 
of  low  concentration,  e.g.,  07  per  cent.  NaCl.  They 
accordingly  precipitate  when  white  of  egg  or  serum 
is  dialysed  against  water,  as  the  salts  present  in  both 
— which-  hold  the  globulins  in  dispersion — are 
removed.  The  process  shows  considerable  hysteresis, 
so  that  the  clear  dialysate  filtered  off  from  the  preci- 


CASEIN   SOLS.  91 

pitate  which   forms  in  the  dialyser  becomes  turbid 
and  deposits  further  globulin  on  standing. 

Casein  is  produced  from  caseinogen,  which  is  the 
most  important  protein  constituent  of  milk,  by 
coagulation  with  acid  or  rennet.  Casein  does  not 
dissolve  in  water  or  in  neutral  salt  solutions,  but 
does  so  in  alkali,  forming  a  well  -  denned  alkali 
caseinate.  The  solution  of  the  latter  is  not 
coagulated  by  heating.  It  shows  all  the  properties 
of  a  typical  emulsoid  sol,  more  particularly  the  rapid 
rise  of  viscosity  with  concentration,  and  the  high 
temperature  coefficient. 


CHAPTER   XIV. 

A  NUMBER  of  emulsoid  sols  with  water,  aqueous 
solutions  and  organic  liquids  as  dispersion  media,  can 
receive  brief  description  only.  Starch  does  not 
disperse  in  cold  water,  but  above  a  certain  tempera- 
ture the  granule's  burst  and  a  bluish  opalescent  sol  is 
formed  ;  in  a  more  concentrated  condition  it  is  known 
to  everybody  as  starch  paste.  One  of  the  most 
striking  characteristics  of  starch  sols  is  their  insta- 
bility, which  manifests  itself,  for  instance,  when  the 
viscosity  is  determined  several  times  in  succession. 
The  later  values  are  much  lower  than  the  earlier  ones, 
the  shearing  undergone  by  the  sol  as  it  flows  through 
the  viscometer  being  sufficient  to  alter  its  character 
markedly.  Separation  of  water  also  sets  in  rapidly  ; 
this  phenomenon  is  specially  marked  with  higher 
concentrations. 

Dextrin,  another  carbohydrate,  forms  sols  in  cold 
water,  and  has  a  slight  but  perceptible  protective 
action  (see  p.  63).  Gum  -  arabic  exhibits  similar 
properties  ;  it  disperses  in  cold  water  without  previous 
swelling,  but  if  the  temperature  is  k;ept  near  o°  C.  a 
period  of  swelling  can  be  seen  to  precede  dispersion. 
Gum-arabic  can  be  "  salted  out  "  reversibly  by  high 
concentrations  of  neutral  salts,  the  process  being 
evidently  merely  a  withdrawal  of  water. 

Among  the  carbohydrates  which  do  not  disperse 
in  water  alone,  but  in  fairly  concentrated  solutions 
of  various  electrolytes,  the  most  important  is  cellu- 
lose. It  is  dispersed  by  solutions  of  cupric  oxide 
in  ammonia  (Schweizer's  reagent),  50  per  cent,  solu- 
tion of  zinc  chloride,  and,  as  has  been  shown  by 


NON-AQUEOUS  SOLS.  93 

v.  Weimarn,  in  concentrated  solutions  of  a  number  of 
highly  soluble  and  hygroscopic  salts,  e.g.,  thiocyanate, 
at  various  temperatures  and  pressures.  The  cupram- 
rrionium  sol  is  one  of  the  materials  for  the  manufac- 
ture of  artificial  silk,  a  coherent  mass  of  cellulose 
being  left  behind  if  the  dispersion  medium  is  removed. 

As  regards  sols  in  organic  media,  the  esters  of 
cellulose,  and  more  particularly  the  nitrates  and 
acetates,  are  of  great  technical  importance.  Some 
of  the  former  dispersed  in  glacial  acetic  acid  and  in 
mixtures  of  ether  and  alcohol  to  form  highly  viscous 
sols  known  as  "  collodion,"  to  which  reference  has 
already  been  made.  Neither  ether  nor  alcohol  alone 
produces  dispersion,  and  the  problem  of  the  "  mixed 
solvent,"  which  has  no  exact  parallel  with  aqueous 
dispersion  media,  is  very  curious  and  deserving  of 
further  study.  Cellulose  acetate  dispersed  in  various 
organic  liquids  plays  an  important  part  as  "  dope  " 
for  impregnating  the  wings  of  aeroplanes. 

India-rubber  forms  sols  with  a  number  of  organic 
solvents,  such  as  benzene,  petroleum  ether,  carbon 
tetrachloride,  etc.  They  all  exhibit  high  viscosity, 
though  this  differs,  for  equal  concentrations  of  the 
same  rubber,  with  different  solvents — a  remark 
which  also  applies  to  the  cellulose  esters. 

The  non-aqueous  sols  have  so  far  received  very 
little  theoretical  study,  and  the  results  of  their 
technical  examination  are  not  accessible.  The 
property  most  investigated  so  far  is  the  viscosity, 
as  the  measurement  of  this  constant  is  simple,  and 
as  experience  has  shown  an  intimate — if  theoretically 
quite  obscure — connection  between  the  viscosity  of 
a  sol  and  certain  properties  of  the  disperse  phase  in 
bulk.  Thus  there  is  a  distinct  parallel  between  the 
viscosity  of  rubber  sols,  made  under  comparable 
conditions,  and  the  elastic  constants  of  the  rubber. 
In  the  case  of  cellulose,  there  is  a  very  remarkable 
persistence  of  some  characteristics  even  when  the 


94  NON-AQUEOUS   SOLS. 

cellulose  has  undergone  such  drastic  treatment  as 
nitration.  It  has  recently  been  shown  (R.  A.  Punter, 
JL  Soc,  Chem.  Ind.,  39,  333T,  1920)  that  the  viscosi- 
ties of  sols  of  various  celluloses  in  cuprammonium 
solutions"  and  the  viscosities  of  sols  of  the  nitrates 
•made  from  them,  exhibit  a  complete  parallelism. 

The  non-aqueous  emulsoids  and  their  gels  still 
form  a  very  promising  field  for  research,  and  it  may 
be  added  that  hardly  any  of  them  have  been  sub- 
mitted to  the  crucial  test  of  dialysis,  so  that  the 
evidence  of  their  colloidal  character  rests  on  infer- 
ences which,  though  no  doubt  correct  in  many  cases, 
are  not  conclusive  in  all.  There  is,  of  course,  no 
difficulty  in  applying  this  test,  given  a  membrane 
of  which  it  has  been  definitely  proved  that  it  is 
permeable  to  the  dispersion  medium  and  that  it 
retains  particles  of  known  size  within  the  colloidal 
range.  Ostwald  and  Wolski  have  recently  shown 
that  such  membranes  can  easily  be  prepared  for 
97  per  cent,  alcohol,  which  retain  mercuric  sulphide 
(Gen.  Disc.  Faraday  Soc.,  October,  1920).  They 
note  the  somewhat  surprising  result  that  the  alcoholic 
solutions,  e.g.,  of  resins,  organic  acids  of  high  mole- 
cular weight,  etc.,  all  show  perceptible  diffusion 
through  a  membrane  of  this  character.  The  exten- 
sion of  these  investigations  to  other  dispersion  media 
is  very  desirable. 


CHAPTER   XV. 

AFTER  this  brief  description  of  individual  emulsoids 
it  is  now  possible  to  discuss  the  general  charac- 
teristics of  emulsoid  sols.  As  regards  their  optical 
properties,  they  are  either  clear  or  slightly  opalescent 
liquids,  most  of  which  show  a  Tyndall  cone  :  the 
turbidity  of  the  latter  depends  on  the  concentration-, 
but  also  on  the  age  and  temperature  of  the  sol 
(gelatin  or  starch  sols).  The  ultra-microscopic 
image  is  much  less  striking  than  that  of  suspensoid 
sols  ;  in  general  an  amicroscopic  light  cone  only  is 
visible,  but  between  certain  limits  of  concentration 
(gelatin)  submicrons  become  visible.  The  absence 
of  submicrons,  however,  does  not  mean  that  the 
sols  are  homogeneous  or  contain  particles  of  amicro- 
scopic size  only  :  it  is  much  more  probable  that  the 
disperse  phase  is  invisible  because  it  is  not  sufficiently 
differentiated  optically  from  the  dispersion  medium, 
i.e.,  the  refractive  index  of  the  highly-hydrated 
particles  may  not  be  very  different  from  that  of  the 
surrounding  liquid. 

Reference  has  already  been  made  several  times  to 
the  viscosity  of  emulsoid  sols.  This  increases  much 
more  rapidly  than  the  weight  of  disperse  phase,  and 
the  temperature  coefficient  of  viscosity  is  con- 
siderably greater  than  that  of  the  pure  dispersion 
medium.  The  two  features  are  well  illustrated  in 
the  curves  A  and  B,  Fig.  10,  which  represent  deter- 
minations made  by  Chick  and  Martin  on  a  sodium 
caseinate  sol.  In  A  the  abscissae  are  concentrations 
in  per  cent.,  while  the  ordinates  are  relative  viscosi- 
ties, that  of  the  dispersion  medium  at  the  same 


96 


VISCOSITY   OF  EMULSOIDS. 


8- 


B 


to         20        SO        40         SO        6O         70         60        9O        IOO     C 

FIG.  10. — VISCOSITY  OF  SODIUM  CASEINATE  SOLS. 

temperature  being  taken  as  unity  ;  in  B  the  abscissae 
are  temperatures,  and  the  ordinates  again  relative 
viscosities,  referred  to  water  at  the  same  temperatures. 


VISCOSITY   OF  EMULSOIDS. 


97 


The  latter  curve  would  of  course  become  a  horizontal 
straight  line  if  the  temperature  coefficient  of  the  sol 
and  of  water  were  the  same.  Fig.  n  shows  vis- 
cosity— concentration  curves  after  Schidrowitz  and 
Goldsbrough  for  rubber  sols  :  the  enormous  increase 


20- 


70- 


0.5  1.0  % 

FIG.  ii. — VISCOSITY  OF  INDIA  RUBBER  SOLS. 

caused  by  concentrations  as  low  as  i  per  cent,  is  well 
shown  by  these  examples. 

The  viscosity  is  not  only  markedly  high  for 
comparatively  low  concentrations,  but  exhibits 
anomalies  absent  in  homogeneous  liquids  with  high 
viscosity,  such  as  glycerin  or  castor  oil.  It  not  only 
varies  with  the  previous  history,  and  the  age  of  the 


98  ANOMALIES   OF   VISCOSITY. 

sol  (silicic  acid,  gelatin),  but  is  to  a  marked  extent 
dependent  on  the  rate  at  which  the  sol  is  sheared. 
These  anomalies  show  themselves  even  in  the 
capillary  viscometer,  but  are  more  easily  studied 
with  other  apparatus  which  permits  a  ready  varia- 
tion of  the  rate  of  shear.  The  first  quantitative 
investigations  were  carried  out  by  Garrett,  who 
employed  the  method  of  measuring  the  logarithmic 
decrement  of  a  disc  oscillating  in  its  own  plane  in 
the  liquid,  from  which  the  viscosity  can  be  deduced 
by  calculation.  He  found  that  the  viscosity  varied 
with  the  amplitude  of  oscillation,  that  it  increased 
rapidly  with  continued  shearing,  etc.  The  condi- 
tions with  the  oscillating  disc  are  too  complicated 
to  permit  interpretation  ;  another  method,  that  of 
measuring  the  deflection  of  cylinder  suspended 
coaxially  in  a  cylindrical  vessel  filled  with  the  liquid 
and  rotated  with  uniform  velocity,  is  free  from  this 
defect,  and  has  been  employed  by  the  author. 
Fig.  12  shows  the  results  obtained  with  the  same 
gelatin  sol,  A,  48  and  B,  24  hours  old  :  the  abscissae 
are  rates  of  shear  (which  are  simply  proportional  to 
the  speed  of  rotation)  in  arbitrary  units,  while  the 
ordinates  are  the  viscosities.  The  viscosity  decreases 
rapidly  with  increasing  rate  of  shear,  and  that  of  the 
sol  48  hours  old  is  throughout  much  higher  than  the 
viscosity  at  24  hours,  a  change  referred  to  already  on 
p.  83.  The  results  are  strong  evidence  of  the  two- 
phase  character  of  these  sols,  and  differ  strikingly 
from  those  obtained  with  homogeneous  liquids  : 
the  viscosity  of  water,  e.g.,  has  been  shown  by  very 
careful  experiments  to  be  independent  of  the  rate 
of  shear  over  the  whole  range  here  used. 

The  osmotic  pressure  of  some  protein  sols  has  been 
studied  by  Lillie,  Moore  and  Roaf ,  Bayliss  and  others. 
Appreciable  osmotic  pressures  have  been  observed 
e.g.,  22-4  mm.  of  mercury  for  a  1-25  per  cent,  albumin 
sol,  6-2  mm.  for  a  1-5  per  cent,  gelatin  sol,  etc.  The 


OSMOTIC   PRESSURE. 


99 


pressure  shows  marked  hysteresis  phenomena,  thus 
on  heating  it  increases  more  rapidly  than  that  of 
true  solutions,  and  on  cooling  does  not  return  to  the 


RATE  OF  SHEAR 

FIG.  12. — VARIATION  OF  VISCOSITY  WITH 
RATE  OF  SHEAR. 

initial  value  for  a  very  considerable  time.  Electro- 
lytes affect  it  markedly  ;  in  studying  their  action  the 
pressure  due  to  the  electrolytes  itself  has  to  be 
balanced  by  providing  the  same  concentration  out- 
side the  semipermeable  membrane  as  inside  it.  Acid 
and  alkali  in  these  conditions  cause  a  very  large 


ioo  EFFECTS   OF  AGEING. 

increase  in  the  osmotic  pressure  of  proteins,  no  doubt 
due  to  the  formation  of  dissociating  protein  salts. 

As  has  been  mentioned  repeatedly,  all  the  pro- 
perties of  many  emulsoid  sols  -undergo  changes 
with  time,  even  if  conditions  are  kept  constant.  The 
property  most  conveniently  measured  and  therefore 
most  extensively  investigated  is  the  viscosity.  In 
most  sols,  e.g.,  silicic  acid  or  gelatin,  the  viscosity 
coefficient  increases  with  age,  and  this  increase  is 
ascribed  to  growing  hydration  or  increasing  size  of 
the  aggregates  of  disperse  phase.  The  latter  process 
undoubtedly  occurs  in  the  early  stages  of  the  dialysis 
of  reaction  mixtures  of  silicic  acid,  since  the  acid 
passes  copiously  through  the  membrane  at  that 
period  (p.  78).  In  other  cases  the  viscosity  decreases 
with  age  ;  a  notable  instance  is  the  eerie  hydroxide 
sol,  which  on  keeping  gradually  loses  its  property  of 
setting  to  a  gel  on  addition  of  electrolytes,  a  change 
which  may  on  good  grounds  be  ascribed  to  pro- 
gressive dehydration.  Rubber  sols  exposed  to  light 
have  been  found  by  several  observers  to  decrease 
in  viscosity  ;  the  cause  suggested — perhaps  on  in- 
sufficient evidence — is  "  depolymerization." 

The  electrical  properties  differ  too  much  in  indi- 
vidual sols  to  allow  of  any  general  summary.  The 
charge  on  the  disperse  phase  is  in  all  cases  intimately 
connected  with  the  degree  of  hydration,  and  its 
removal  affects  the  system  largely,  if  not  entirely, 
through  the  change  in  hydration. 


CHAPTER  XVI. 

WE  have  now  to  consider  a  number  of  substances 
showing  peculiarities  differing  markedly  from  those 
of  the  more  typical  emulsoids  described  so  far, 
and  forming  a  more  or  less  gradual  transition  to 
true  solutes.  Among  these  the  soaps,  more  par- 
ticularly the  sodium  and  potassium  salts  of  the 
fatty  acids,  take  the  first  place  as  regards  theo- 
retical interest  and  technical  importance.  Our  know- 
ledge of  the"3se|bodies  rests  to  a  great  extent  on 
the  exhaustive  investigations  of  McBain  and  his 
pupils. 

In  alcohol  the  soaps  form  true  solutions  showing 
the  general  characteristics  of  solutions  of  a  simple 
and  rather  weak  electrolyte.  In  water  they  exhibit 
a  marked  anomaly,  inasmuch  as  they  possess  the 
conductivity  of  a  moderately  strong  electrolyte  but 
an  osmotic  pressure  very  much  lower  than  would 
account  for  this  conductivity.  The  deficiency  in 
osmotic  pressure  must  be  ascribed  to  the  formation, 
from  a  portion  of  the  dissolved  soap,  of  colloidal 
particles  or  aggregates  too  large  to  exert  an  appre- 
ciable osmotic  pressure  ;  that  measured  is  practically 
due  to  K  or  Na  ion  only.  The  conductivity,  however, 
is  much  greater  than  corresponds  to  this  cation  con- 
centration, and  to  reconcile  this  discrepancy  it  is 
necessary  to  assume  that  the  colloidal  particles  carry 
each  a  large  number  of  charges  so  that  their  mobility 
becomes  that  of  an  ordinary  ion  in  spite  of  their  large 
size.  The  constitution  which  McBain  finally  ascribes 
to  these  colloidal  anions  is  that  they  are  aggregates 


102  ,     THE  DYES. 

of  undissociated  soap  molecules,  a  number  of  fatty 
acid  anions  and  a — probably  large — number  of  water 
molecules.  The  reader  will  be  struck  by  the  analogy 
with  proteins  (p.  go). 

In  other  respects  soap  solutions  exhibit  fairly 
marked  emulsoid  behaviour  ;  on  cooling  or  on  the 
addition  of  salts  they  set  to  clear  gels  or  to  opaque 
masses  which  McBain  differentiates  from  the  former 
as  "  curds."  They  are  also  "  salted  out  "  by  high 
concentrations. 

The  dyestuffs  exhibit  a  very  great  variety  of 
behaviour.  Some,  like  eosin  or  methylene  blue",  are 
in  true  solution,  show  no  heterogeneity  in  the  ultra- 
microscope  and  diffuse  rapidly.  Others,  like  Congo 
Red,  and  even  more  strikingly  Congo  Rubin,  which 
latter  has  been  studied  in  great  detail  by  Wo. 
Ostwrald,  behave  almost  like  typical  suspensoids. 
Both  are  sodium  salts  of  a  blue  acid  and,  therefore, 
turn  blue  on  the  addition  of  free  acid.  Congo  Rubin 
also  turns  violet  or  blue  on  the  addition  of  neutral 
salts  and  even  of  Ba(OH)2  and  eventually  pre- 
cipitates as  a  purple  or  blue  coagulum,  very  much 
like  a  gold  sol.  The  effect  of  the  valency  of  the  cation 
is  quite  marked  ;  with  many  electrolytes  in  suitable 
concentrations  the  colour  change  is  reversible  by 
mere  heating,  i.e.,  the  purple  sol  turns  red  and 
remains  so  on  cooling.  Congo  Rubin  sol  can  be 
protected  by  the  same  emulsoids  as  gold  sol,  although 
the  order  is  not  quite  the  same  ;  it  exhibits  the 
further  striking  peculiarity  that  protection  may  be 
effected  after  the  colour  change  has  taken  place  (see 
p.  65).  On  the  addition  .of  protective  colloid  to  a  sol 
turned  purple  by  electrolyte  the  colour  changes 
back  to,  and  remains,  red. 

The  osmotic  pressure  of  the  related  Congo  Red 
has  been  investigated  by  Bayliss.  This  dye  does  not 
diffuse  through  parchment  and  behaves  towards 
electrolytes  like  a  negatively  charged  suspensoid  ; 


THE   DYES.  103 

nevertheless  the  sol  has  an  osmotic  pressure  only 
slightly  less  than  that  calculated  from  the  (simple) 
formula  weight  and  is  a  good  conductor  in  high  dilu- 
tion. The  blue  dye  acid,  on  the  contrary,  has  an 
abnormally  low  osmotic  pressure. 

Night  Blue,  on  the  other  hand,  is  positively  charged 
and  recalls  the  hydroxide  sols  in  showing  some 
emulsoid  characteristics.  Concentrated  solutions  are 
viscous,  and  the  viscosity  varies  with  age  and  with 
the  concentration  of  electrolytes  present.  It  is 
hardly  necessary  to  add  that  dye  sols  with  opposite 
electric  charges  precipitate  each  other,  and  that  the 
phenomenon  agrees  with  the  rule  given  above  (p.  62), 
viz.,  precipitation  is  complete  when  the  twro  sols  are 
mixed  in  certain  ratios,  but  does  not  occur  if  either 
is  in  excess. 

Many  dyes  which  are  optically  void  in  the  ultra- 
microscope  show  particles  after  the  addition  of 
neutral  salts  (Fuchsin  with  NaCl)  or  more  frequently 
of  acid  or  alkali,  the  dye  acid  or  dye  base  liberated 
being  colloidal  while  the  dye  itself  is  not.  A  very 
exhaustive  ultra-microscopic  investigation  of  fifty 
dyes  of  the  indicator  class  has  been  carried  out  by 
Wo.  Ostwald,  from  which  it  appears  that  in  prac- 
tically all  cases  either  the  base  or  the  acid,  if  not 
the  dye  itself,  shows  particles  or  at  least  a  diffuse 
light  cone. 

In  alcohol  or  other  organic  solvents  many  dyes 
show  true  solubility  and  exhibit  normal  osmotic 
pressure  and  molecular  weight. 

A  number  of  substances,  frequently  described  as 
"  semi-colloids  "  provide  a  gradual  transition  to  true 
solutions.  Among  them  are  the  various  products  of 
cleavage  of  proteins.  Protalbic  and  lysalbic  acid, 
for  instance  (see  p.  64),  show  a  measurable  depression 
of  the  freezing  point,  and  the  latter  diffuses  percep- 
tibly through  parchment  ;  on  the  other  hand,  their 
solutions  and  those  of  their  alkali  salts  are  turbid 


104  SEMI-COLLOIDS. 

and  become  viscous  in  high  concentrations.  Peptone 
solutions  behave  similarly,  and  the  chief  properties 
which  link  them  to  the  emulsoids  are  slight  turbidity 
and  the  syrup  consistence  which  they  assume  with 
increasing  concentration. 


CHAPTER  XVII. 

REFERENCE  has  already  been  made  in  several  of 
the  preceding  chapters  to  the  changes  which  sols 
like  those  of  gelatin  and  agar  undergo  on  cooling,  or 
which  take  place  in  silicic  acid  sol  on  ageing  or  on  the 
addition  of  electrolytes.  The  sols  in  these  conditions 
set  to  coherent  gels  without — at  any  rate  immediate 
—loss  of  dispersion  medium,  and  notwithstanding 
the  very  large  amount  of  the  latter  present  in  some 
of  them — 2  per  cent,  agar  makes  a  very  stiff  gel— 
they  possess  some  of  the  properties  of  solids.  The 
term  "  gel "  was  applied  by  Graham  to  all  the 
products  of  transformation  of  sols,  but  in  the 
following  is  restricted  to  such  as  have  been  quoted 
above  and  similar  systems,  in  which  no  appreciable 
segregation  of  the  phases  takes  place.  It  may  be 
well  to  mention  here  that,  even  in  its  restricted 
meaning,  the  term  is  not  capable  of  strict  definition  : 
by  the  majority  of  authors  it  is  applied  to  transparent 
or  opalescent  masses  containing  a  large  percentage 
of  liquid  which  retain  their  shape,  offer  some  resis- 
tance to  mechanical  deformation  and  show  no 
microscopic  structure. 

The  gels  are  generally  divided  into  "  elastic  "  and 
"  rigid  "  gels.  Gelatin,  agar,  the  rubber  and  cellulose 
nitrate  gels  belong  to  the  former  class,  while  silicic 
acid  gel  is  the  principal  representative  of  the  latter. 
The  distinction  is  not  altogether  happy,  since  a 
moderately  concentrated  silicic  acid  gel  can  be  felt 
to  vibrate  when  the  vessel  containing  it  is  struck, 
but  they  are  descriptive  of  some  properties  which  will 
be  understood  when  the  individual  examples  have 
been  described. 


io6  SILICIC   ACID   GEL. 

Silicic  acid  is  the  only  rigid  gel  which  has  been 
studied  exhaustively.  It  is  obtained  in  its  pure 
form  by  allowing  a  well  dialyzed  sol  to  set,  either 
spontaneously  or  by  small  additions  of  ammonia, 
a  carbonate  or  a  phosphate.  The  sol  becomes 
bluish  and  sets  to  a  jelly,  the  opalescence  of  which 
increases  for  some  time  after  setting  is  apparently 
complete,  i.e.,  after  the  vessel  containing  the  gel  can 
be  turned  upside  down.  No  appreciable  segregation 
of  water  occurs  immediately  nor,  if  the  gel  is  dilute, 
even  after  some  time.  With  concentrated  gels, 
however  (5  per  cent.  SiO2  or  more),  a  very  striking 
change  shows  itself  within  a  few  minutes.  The  gel, 
which  appears  perfectly  dry  immediately  on  setting, 
becomes  covered  with  drops  of  liquid  which  rapidly 
increase  in  size,  and  at  the  same  time  liquid  is 
exuded  between  the  gel  and  the  walls  of  the  vessel. 
After  twenty  or  thirty  minutes  this  has  progressed 
so  far  that  the  gel  will  slip  out  of  the  containing  vessel 
when  this  is  inverted.  This  segregation  of  liquid— 
which  also  occurs  in  all  the  elastic  gels  in  suitable  con- 
centrations— had  already  been  observed  by  Graham, 
who  called  it  "  Syneresis  "  ;  it  is  a  phenomenon 
probably  of  great  importance  in  nature  which  deserves 
more  study  than  it  has  received  up  to  the  present. 

The  pure  gel,  if  left  in  air,  rapidly  loses  water,  even 
at  ordinary  temperature,  and  dries  to  a  glass-like 
transparent  mass  which  is  not  opalescent.  It  still 
retains  several  moles  of  water  per  mole  of  SiO2, 
which  can  be  removed  by  drying  over  sulphuric 
acid.  The  course  of  dehydration  has  been  studied 
by  van  Bemmelen  in  a  series  of  classical  investiga- 
tions, prolonged  in  one  instance  for  two  years.  The 
gels  were  kept  in  desiccators  containing  sulphuric 
acid-water  mixtures  of  known  vapour  pressures  and 
were  weighed  at  frequent  intervals.  By  passing  the 
dry  gel  through  the  series  of  desiccators  in  the 
inverse  order  the  course  of  rehydration  was  likewise 


THE  DEHYDRATION   CURVE. 


107 


i.o 


0.5 


4  8  12 

FIG.  13. — DEHYDRATION  OF  SILICIC  ACID  GEL. 

investigated.  The  results,  covering  the  range  from 
3  to  o  moles  of  H2O  per  mole  of  SiO2,  are  plotted 
in  Fig.  13,  in  which  the  abscissae  are  the  vapour 


io8          THE   DEH\DRATION   CURVE. 

pressures  in  millimetres  of  mercury,  the  ordinates 
the  water  concentration  in  moles,  and  the  arrows 
indicate  the  direction  of  the  process,  viz.,  those 
pointing  towards  the  left  dehydration,  and  those 
pointing  towards  the  right  rehydration. 

The  first  point  to  be  gathered  from  the  examination 
of  the  diagram  is  that  there  are  no  definite  hydrates, 
but  a  continuous  loss  of  water  with  an  equilibrium 
corresponding  to  every  given  vapour  pressure.  In 
this  respect  the  gel  differs  radically  from  crystals  with 
water  of  crystallization.  Copper  sulphate,  e.g., 
crystallizes  with  five  molecules  of  water,  four  of 
which  are  given  off  at  100°  C.  and  the  fifth  at  200°. 
A  dehydration  curve  plotted  with  the  temperatures 
as  abscissae  and  the  water  contents  as  ordinates 
would  therefore  be  discontinuous  and  would  consist 
of  a  succession  of  straight  lines. 

We  meet  here,  for  the  first  time,  a  class  of  com- 
pounds quite  definite  under  given  conditions,  but 
in  which  the  ratio  of  the  constituents  can  change 
continuously  and  not  only  by  steps  corresponding  to 
simple  stoichiometric  ratios.  We  shall  have  an  oppor- 
tunity of  considering  such  systems  in  connection 
with  the  laws  of  absorption. 

The  gel  undergoes  a  very  striking  change,  which 
must  be  mentioned  specially,  as  throwing  light  on 
the  structure  of  the  dry  gel.  At  A  the  gel  is  trans- 
parent, while  at  B  it  becomes  white  and  opaque, 
somewhat  resembling  porcelain.  On  still  further 
drying,  however,  the  gel  becomes  clear  again  and  is 
transparent  in  the  last  part. 

The  process  of  dehydration  is  reversible  in  parts, 
but  rehydration  shows  hysteresis.  The  curves 
branching  off  to  the  right  at  275  and  0-5  moles  show, 
according  to  the  arrows,  the  course  of  rehydration 
begun  at  those  points,  and  of  a  second  dehydration. 

The  completely  dried  gel  imbibes  organic  liquids 
and  remains  clear.  If  placed  in  water  or  even  a 


STRUCTURE  OF  DRY   GEL.  109 

moist  atmosphere,  it  begins  to  crack  violently  and 
disintegrates  into  small  fragments.  Investigations 
to  elucidate  its  structure  have  been  carried  out, 
principally  by  Zsigmondy  and  his  pupils  Bachmann 
and  Anderson,  with  the  following  results  : — 

Since  the  gel  rapidly  imbibes  liquid,  it  must  be 
porous  and  the  pores  must  communicate  in  all 
directions.  A  limit  to  the  size  of  the  pores  as  well 
as  to  the  thickness  of  the  walls  between  them  is  fixed 
by  optical  considerations  ;  since  the  gel,  whether 
filled  with  air  or  with  an  organic  liquid,  is  transparent 
except  at  B,  both  the  dimensions  mentioned  must 
be  small  compared  with  the  wave  length  of  light,  as 
otherwise,  in  consequence  of  the  difference  in  refrac- 
tive index  of  walls  and  contents,  the  whole  would 
appear  turbid.  The  actual  size  of  the  pores  can  be 
calculated  with  a  reasonable  degree  of  approximation 
from  the  vapour  pressure  of  the  liquid  contained  in 
the  gel.  As  is  well  known  (see  "  Surface  Tension  and 
Surface  Energy  ")  the  vapour  pressure  over  the 
concave  meniscus  formed  by  a  wetting  liquid  in  a 
capillary  is  lower  than  that  over  a  plane  surface  of 
the  same  liquid,  and  the  radius  of  the  capillary  can 
be  calculated  from  the  two  vapour  pressures.  The 
value  thus  arrived  at  is  about  3  pp.  While  the  gel 
is  transparent  when  filled  either  with  liquid  or  with 
air,  the  formation  of  any  spaces  containing  air  in  a 
gel  still  largely  filled  with  liquid  would  cause  an 
optical  heterogeneity  sufficient  to  produce  turbidity, 
and  this  is  Zsigmondy's  explanation  of  the  sudden 
opacity  shown  at  the  point  B  of  the  dehydration 
curve. 

The  structure  of  the  fresh  silicic  acid  gel  cannot  be 
determined  by  such  methods  and  is  still  a  matter  of 
controversy,  to  which  it  will  be  more  convenient  to 
refer  when  elastic  gels  have  been  described. 
Examination  by  X-ray  interference,  carried  out  by 
Scherrer  (R.  Zsigmondy,  "  Kolloidchemie,"  3rd 


no  VARIOUS   RIGID   GELS. 

edition,  1920,  p.  407)  has  not  revealed  any  trace 
of  crystalline  structure,  although  this  shows  itself  in 
dry  and  particularly  in  ignited  gels. 

Another  gel  of  the  same  type  which  has,  however, 
received  very  little  attention  is  that  of  eerie  hydroxide 
(see  p.  68) .  It  is  a  pale  yellow  and,  according  to 
the  extent  to  which  it  has  been  dialyzed,  and  to  the 
rapidity  of  coagulation,  either  quite  clear  or  opales- 
cent. It  dries  to  a  reddish  brown  translucent  mass 
at  moderate  temperatures,  but  the  change  has  not 
been  investigated  in  detail.  The  fresh  gel  is  remark- 
able for  the  ease  with  which  it  can  be  peptized  again  ; 
contact  with  fresh  sol  or  the  addition  of  i  or  2  drops 
of  N/i  nitric  acid  are  sufficient  to  transform  the  gel 
back  into  sol  after  a  few  hours. 

Another  rigid  gel  which  deserves  further  study  is 
that  of  gelatin  hardened  with  formaldehyde.  If  an 
ordinary  gelatin  gel  is  exposed  to  formaldehyde  gas 
or  submerged  in  the  aqueous  solution,  it  gradually 
becomes  much  harder,  very  brittle  and  dries  to  a 
friable  mass.  The  gel  so  treated,  unlike  the  natural 
gelatin  gel,  no  longer  swells  in  water.  Prolonged 
submersion  in  water  which  is  changed  at  intervals 
removes  the  greater  part  of  the  formaldehyde  and 
the  gel  reverts  more  or  less  completely  to  its  original 
condition. 

Diffusion  and  reactions  in  rigid  gels,  more  especially 
in  silicic  acid  gel,  have  not  received  as  much  attention 
as  the  same  phenomena  in  elastic  gels,  and  will 
receive  mention  when  the  latter  are  described. 


CHAPTER   XVIII. 

THE  typical  representative  of  the  elastic  gels  is 
that  of  gelatin,  which  has  been  studied  in  various 
aspects  by  a  large  number  of  observers.  The  raw 
material  or  dry  gel  occurs  in  commerce  in  sheets 
(generally  showing  the  diamond-shaped  markings  of 
the  wire  netting  on  which  they  have  been  dried),  as 
powder  and  in  the  form  of  thin  shreds.  The  air-dry 
material  contains,  in  ordinary  atmospheric  conditions, 
from  10  to  15  per  cent,  of  water,  most  of  which  can  be 
removed  by  drying  at  100°  C.  If  the  dry  gel  is  placed 
in  an  atmosphere  saturated  with  moisture,  it  takes 
up  water,  rapidly  at  first  and  then  more  slowly, 
until  equilibrium  is  attained.  In  an  experiment  by 
Schroeder  the  weight  of  water  thus  taken  up  by  a 
gelatin  plate  weighing  dry  0-904  gm.  was  0-37  gm. 
after  eight  days,  after  which  it  remained  constant. 
The  plate  was  then  submerged  in  water  at  the  same 
temperature  and  took  up  a  further  5-63  gm.  in  the 
first  hour.  The  process  is  accompanied  by  a  con- 
siderable increase  in  the  volume  of  the  gel — a  change 
which  in  the  absence  of  a  special  term  is  usually 
described  as  "  swelling  " — and  the  process  continues 
for  a  considerable  time,  which  depends  on  the  linear 
dimensions  of  the  piece  investigated.  At  a  given 
temperature  equilibrium  is  finally  reached. 

If  the  fully  swollen  gel  is  now  placed  in  a  dry 
atmosphere,  it  rapidly  loses  water  with  a  corre- 
sponding decrease  in  volume.  A  great  portion  of  the 
water  is  held  quite  loosely,  and  evaporation  from  the 
gel  proceeds  at  first  as  rapidly  as  from  a  water 
surface  of  the  same  area  (Pauli).  An  equilibrium 


ii2  HEAT  OF  SWELLING. 

determined  by  the  vapour  pressure  is  finally  attained 
in  drying. 

While  the  gel  imbibing  water  swells,  the  total 
volume,  gel  plus  water,  decreases,  i.e.,  the  process  is 
accompanied  by  compression,  as  occurs,  of  course, 
also  in  many  cases  of  true  solution.  While  the  com- 
pression is  easily  demonstrated  qualitatively,  it  has 
hardly  been  investigated  quantitatively  and  work  in 
this  direction  would  be  of  great  interest.  At  present 
it  is  not  even  possible  to  say  which  of  the  components, 
if  not  both,  undergoes  compression,  though  the  effect 
of  the  lyotropic  series  of  anions,  which  will  be 
described  shortly,  makes  it  probable  that  the  water 
is  responsible  for  at  least  part  of  it.  In  any  case  it 
follows  from  compression  taking  place  in  either  phase 
that  the  process  of  swelling  must  be  accompanied  by 
the  liberation  of  heat.  This  is  indeed  the  case,  and 
calorimetric  measurements  have  been  made  on 
gelatin  as  well  as  on  other  substances  which,  like  it, 
swell  in  water.  The  following  figures  are  given  by 
Wiedemann  and  Luedeking  :— 

Heat  developed  on 
swelling,  in  gramme-calories 
Gel.  per  gm.  of  gel. 

Gelatin           . .  , .  5-7 

Starch             . .  . .  . .  6-6 

Gum-arabic    . .  . .  . .  9-0 

Gum-tragacanth  .  .  .  .  10-3 

WTiile  the  total  volume,  gel  +  water,  decreases, 
that  of  gel  4-  imbibed  water  increases  and  the  gel 
swells.  The  swelling  can  be  demonstrated  and 
measured  with  some  degree  of  accuracy  in  a  variety 
of  ways.  One  method  consists  in  placing  in  test 
tubes  of  equal  diameter  weighed  amounts  of  the 
powdered  gel,  adding  the  dispersion  medium  and 
measuring  the  increase  in  height  of  the  layer  of  gel. 
With  gelatin  another  method  may  be  used.  Circular 
discs  are  cut  from  a  gelatin  sheet  or  foil  of  uniform 


THE  (EDOMETER. 


thickness  and  submerged  in  the  liquids  to  be 
examined  ;  as  the  discs  swell  in  all  three  dimensions, 
the  volume  increases  with  the  cube  of  radius,  which 
is  most  conveniently  measured. 

It  is  well  known  that  gels,  and  the  complicated 
organic  tissues  composed  of  them,  exert  considerable 
forces  when  swelling.  To  measure  these  it  is,  of 
course,  necessary  to  confine  the  gel  in  such  a  way 
that  the  water  has  access  to  it, 
but  is  not  implicated  in  the  • 

volume  changes.    An  apparatus 
in   which   this    is    possible    was  | 

designed  by  Reinke  and  called 
by  him  "  (Edometer  "  :  a  dia- 
grammatic section  is  shown  in 
Fig.  14.  He  examined,  not  a 
simple  gel,  but  dry  discs  of 
Laminaria,  a  sea  weed.  These 
were  placed  in  the  cylinder  A, 
which  they  fitted  exactly,  and 
weighted  by  a  piston  provided 
with  numerous  fine  perforations, 
through  which  water  contained 
in  the  upper  part  of  the  cylinder 
could  reach  the  discs  freely.  As 
the  latter  could  expand  only 
axially  the  increase  in  volume 
was  simply  proportional  to  the 
linear  displacement  of  the  piston.  The  table  on 
page  114  gives  the  pressure  on  the  piston  in  atmo- 
spheres (kilogramme  per  square  centimetre)  and  the 
percentage  increase  in  volume  which  the  imbibition 
of  water  produces. 

This  table  also  illustrates  strikingly  the  large 
amounts  of  energy  that  enter  into  the  process.  Even 
against  the  enormous  pressure  of  over  41  atmospheres 
the  gel  still  expands  16  per  cent.,  while  with  a  pres- 
sure of  i  atmosphere  the  expansion  amounts  to  330- 


FIG.  14.  —  DIAGRAM- 
MATIC SECTION  OF 
(EDOMETER. 


ii4          DISTORTION   BY   SWELLING. 

per  cent.  In  other  words,  if  a  cube  of  gel  having 
an  edge  of  i  cm.  is  allowed  to  swell  in  one  dimension 
only  (as  in  the  cedometer)  to  saturation,  it  will  lift 
i  kg.  3-3  cm.  Conversely  it  becomes  clear  what 
enormous  pressures  are  necessary  to  remove  the  last 
traces  of  water  from  a  gel,  as  the  present  example 
still  retains  16  per  cent,  under  a  pressure  of  41 
atmospheres. 

The  process  of  swelling  is  naturally  controlled  by 
the  diffusion  of  the  liquid  to,  and  into,  the  gel,  and 
this  dependence  becomes  marked  if  larger  bodies  of 
gel  are  allowed  to  swell.  The  outer  layers  in  imme- 

Percentage  increase 
Pressure  in  atm.  of  volume. 

41-2    .  .          .  .          .  .          .  .       16 

31-2     ......  -.         23 

21-2  .  .  .-35 

n-2  .  .          .  .          .  .          .  .        8q 

7'2  ........       97 

3'2  ........     205 


I'O      ........       330 

diate  contact  with  liquid  swell  rapidly  ;  to  reach  the 
interior,  the  liquid  has  to  diffuse  through  an  increas- 
ing thickness  of  swollen  gel  and  reaches  a  gradually 
decreasing  volume  of  dry  gel.  The  sum  of  these 
effects  is  that  the  increase  in  weight  becomes  slower 
with  time.  A  body  of  gel,  unless  it  is  of  a  simple 
shape  with  more  cr  less  uniform  radius  of  curvature, 
does  not  remain  similar  to  itself  during  swelling,  and 
may  undergo  very  considerable  distortion  if  its  shape 
gives  very  unequal  facility  for  diffusion,  i.e.,  if  it 
has  points,  edges,  or  portions  differing  greatly  in 
thickness. 

The  drying  of  a  gel  is  the  converse  and  is  controlled 
by  the  diffusion  of  the  liquid  through  the  gel  to  the 
surface.  Edges  or  points  dry  more  rapidly  than  the 


SOLUTES  AND   SWELLING.  115 

rest,  contract  and  eventually  become  much  more 
rigid  than  larger  surfaces,  in  which  drying  proceeds 
more  slowly.  This  again  may  lead  to  considerable 
distortion.  An  illustration  is  given  in  Fig.  15,  which 
gives  diagrammatic  profiles  of  three  stages  in  the 
drying  of  a  cylinder  of  10  per  cent,  gelatin  gel. 

The  swelling  of  gelatin  and  of  other  gels  is  very 
considerably  affected  by  substances  dissolved  in  the 
water,  and  most  strikingly  by  salts  of  the  lyotropic 
series.  In  citrate,  tartrate  or  sulphate  solutions,  the 
gel  swells  less  than  in  pure  water,  while  in  iodides  and 
thiocyanates  swelling  is  considerably  increased  ;  in 
sufficiently  concentrated  solutions  of  the  latter  it 


FIG.   15. — SUCCESSIVE   STAGES   IN  DRYING  OF 
GELATIN  GEL  CYLINDER. 

disperses  even  in  the  cold.  The  action — as  far  as  an 
analogy  is  possible — is  parallel  with  that  on  sols  (see 
p.  84)  ;  the  anions  which  raise  the  viscosity  and  the 
setting  point  reduce  the  amount  imbibed,  while 
those  which  lower  the  viscosity  and  the  setting 
temperature  increase  it.  A  similar  effect  on  the 
elastic  properties  of  the  gel  will  be  referred  to 
shortly. 

Gelatin  gels  show  perfect  elasticity  under  moderate 
stress  applied  for  a  short  time.  If  the  application 
is  prolonged,  the  stress  gradually  disappears,  i.e.,  the 
gel  relaxes  and  retains  the  shape  to  which  it  has  been 
deformed.  Determinations  of  the  modulus  of  elasti- 
city for  tension  have  been  made  by  various  investiga- 
tors. A  table  due  to  Leick  is  here  given.  C  is  the 

8— -2 


n6  ELASTICITY   OF   GELS. 

gelatin  concentration  in  per  cent.,  E  the  modulus 
for  tension  in  grammes  per  square  millimetre. 

C.  E  (gm./mm.2).     E/C2. 

io-o  . .  . .  2-42  2-4 

18-6  . .  . .  978  2-8 

30-0  ..  ..  15-45  i-7 

45-0  ..  29-44  1-5 

For  the  lower  concentrations  the  modulus  is  approxi- 
mately proportional  to  the  square  of  the  gelatin  con- 
centration, as  appears  from  the  last  column. 

The  gels  do  not  attain  their  full  strength  imme- 
diately on  setting,  but  the  modulus  increases  for 
some  time  and  does  not  attain  its  maximum  value 
until  three  or  four  hours  afterwards.  The  volume 
remains  practically  constant  during  deformation,  a 
behaviour  which  is  not  surprising  in  view  of  the  large 
amount  of  liquid  contained  in  the  gel.  The  influence 
of  various  solutes  has  also  been  studied  ;  chlorides 
and  nitrates  lower  the  modulus  markedly,  while 
sulphates  raise  it  slightly  and  cane  sugar  consider- 
ably. These  effects  again  show  the  same  order  as 
the  action  of  the  same  solutes  on  the  setting  points 
of  sols  and  the  swelling  of  gels. 

Gels  free  from  strain  are  isotropic,  i.e.,  they  have 
the  same  coefficient  of  thermal  expansion,  modulus 
of  elasticity  and  refractive  index  in  all  directions. 
The  coefficient  of  expansion  in  these  circumstances 
is  practically,  and  over  a  fair  range,  that  of  the  liquid 
contained  in  the  gel.  The  refractive  index  is,  like 
that  of  sols,  a  linear  function  of  the  gelatin  concen- 
tration. If  a  gel  is  strained,  it  becomes  birefringent  ; 
owing  to  the  difference  in  the  rate  of  setting  in 
ordinary  vessels,  it  is  difficult,  without  considerable 
precautions,  to  obtain  bodies  of  gel  entirely  free  from 
strain.  Uneven  drying  produces  similar  effects  :  if 
a  small  cube  of  10  per  cent,  gelatin  gel  is  observed 
with  polarized  light  while  drying  at  ordinary  tern- 


SYNERESIS.  117 

perature,  the  first  signs  of  double  refraction  generally 
show  within  about  five  minutes,  and  the  strain  due 
to  the  shrinking  of  the  edges  is  very  marked  after 
20  or  30  minutes. 

The  properties  of  gels  other  than  that  of  gelatin 
have  so  far  received  very  little  investigation,  and 
the  non-aqueous  systems  in  particular  would  repay 
study.  One  phenomenon  already  described  in  con- 
nection with  silicic  acid  gel — syneresis — is  common 
also  to  elastic  gels,  though  the  concentration  at 
which  it  is  most  marked  differs  considerably.  While 
silicic  acid  gel  shows  marked  syneresis  at  high  con- 
centrations, with  gelatin  and  agar  gels  it  is  best 
observed  in  dilute  gels.  Agar  gels  containing  i  or 
1-5  per  cent,  of  the  dry  substance,  on  standing  for  a 
few  hours  segregate  a  perceptible  quantity  of  liquid 
— a  phenomenon  well  known  in  its  bacteriological 
use  as  a  culture  medium.  The  syncretic  liquid  is  in 
no  case — including  silicic  acid — the  pure  dispersion 
medium,  but  always  contains  a  small  amount  of 
disperse  substance.  If  the  dispersion  medium  is  a 
solution,  the  solute  is  likewise  found  in  the  segregated 
liquid,  but  the  concentrations  in  the  latter  and  in  the 
gel  are  generally  different.  Some  gels  in  organic 
media,  e.g.,  viscose  (cellulose  xanthate)  or  india- 
rubber  vulcanized  in  solution,  show  syneresis  even 
more  strikingly  than  do  the  aqueous  gels  ;  in  viscose 
the  liquid  segregated  after  some  months  may  amount 
to  50  per  cent,  of  the  original  volume. 

The  phenomenon  has  received  singularly  little 
attention  since  it  was  first  observed  by  Graham,  but 
in  many  of  his  recent  publications  Wo.  Ostwald  has 
insisted  on  its  great  theoretical  importance  and  on 
its  possible  bearing  on  problems  like  that  of  secretion 
from  glands,  etc.  This  property  of  gels — as,  indeed, 
most  of  them — still  offers  a  large  and  promising  field 
for  research. 


CHAPTER  XIX. 

THAT  dissolved  substances  diffused  into  and  out  of 
gels  was  known  already  to  Thomas  Graham,  and  is 
a  fact  familiar  to  every  photographer,  as  the  various 
processes  of  developing,  fixing  and  toning  are  made 
possible  only  by  the  diffusion  of  the  respective 
solutions  into  the  gelatin  gel  containing  the  silver 
salt.  Graham  also  observed  the  difference  in  the 
behaviour  of  true  and  colloidal  solutions — which 
latter  do  not  diffuse  perceptibly  into  gels — noticed 
that  solutions  of,  e.g.,  sodium  chloride  diffused  in 
gelatin  gel  with  almost  the  same  velocity  as  in  water 
alone,  and  made  use  of  this  observation  to  determine 
diffusion  constants  in  gels,  thus  avoiding  the  diffi- 
culties caused  in  liquids  by  convection  currents,  etc. 

The  difference  between  true  and  colloidal  solutions 
can  be  easily  demonstrated  by  filling  test  tubes  to 
about  two-thirds  of  their  height  with  i  per  cent,  agar 
sol,  allowing  it  to  set  and  then  pouring  on  the  solu- 
tions to  be  examined.  While  solutions  of,  say, 
potassium  dichromate,  copper  sulphate  or  some  dyes 
like  methyl  violet  will  be  found  to  advance  rapidly 
into  the  gel,  sols  of  gold,  or  of  dyes  like  Night  Blue, 
Congo  Red,  etc.,  will  not  penetrate  into  it. 

Further  investigation  since  Graham's  time  has 
shown  that  the  rate  of  diffusion  is  unaffected  only 
in  dilute  gels,  but  is  materially  slower  in  more 
concentrated  gels  than  in  the  pure  liquids.  The 
permeability  of  gels  to  solutes  can  be  altered,  like 
all  their  other  properties,  by  various  dissolved  sub- 
stances. Without  going  into  details  it  may  be  said 
that  the  study  of  diffusion  is  by  no  means  simple  : 


DIFFUSION   IN   GELS.  119 

even  with  strongly  coloured  solutions  like  those  of 
dichromates  it  is  not  easy  to  fix  the  actual  boundary, 
since  there  is  a  steady  gradient  to  zero  concentra- 
tion. An  indicator  can  be  used  only  if  it  does  not 
itself  diffuse  appreciably,  as  otherwise  the  results 
are  vitiated  by  its  movement  in  the  opposite  direc- 
tion :  dyes  of  the  indicator  type  generally  fulfil  this 
condition.  Another  method  consists  in  so  arranging 
the  body  of  gel — for  example,  cylinders  cast  in  test 
tubes  lined  with  parchment — that  it  can  be  examined 
over  its  entire  surface  :  it  is  withdrawn  at  intervals 
and  the  extent  of  diffusion  determined  by  "  spotting  " 
with  a  suitable  reagent.  Experiments  of  this 
description  have  been  carried  out  by  Bechhold  and 
Ziegler,  who  found  that  the  addition  to  a  gelatin 
gel  of  sodium  sulphate,  glucose,  alcohol  or  glycerin 
retarded  diffusion,  while  urea,  iodides  and  chlorides 
accelerated  it.  These  effects  are  again  parallel  with 
the  changes  in  setting  point  and  elastic  properties 
caused  by  the  same  solutes. 

Since  a  solute  will  diffuse  into  a  gel  containing 
another  substance  in  solution,  it  is  possible  to  pro- 
duce reactions  in  gels,  and  these  are  of  very  con- 
siderable interest,  since  the  innumerable  reactions 
which  occur  in  organisms  very  largely  proceed  in 
media  with  the  properties  of  gels.  Two  cases  have 
to  be  distinguished  :  reactions  producing  one  (or 
two)  insoluble  reaction  products,  and  those  in  which 
all  the  products  of  the  reaction  are  soluble.  In  the 
latter  case,  which  has  received  comparatively  little 
attention  so  far,  diffusion  takes  place  in  both  direc- 
tions, i.e.,  the  solute  contained  in  the  gel  diffuses 
into  the  aqueous  solution,  and  vice  versa,  irrespective 
of  the  concentrations  and  osmotic  pressures  of  the  two 
reaction  components.  If,  for  instance,  a  test  tube 
is  partly  filled  with  agar  gel  containing  copper 
sulphate,  and  a  solution  of  ammonia  poured  on  top 
of  the  gel,  both  the  latter  and  the  supernatant 


120  REACTIONS   IN   GELS. 

aqueous  solution  turn  deep  blue  owing  to  the  forma- 
tion of  copper  oxide-ammonia  (S.  C.  Bradford, 
Bioch.  //.,  14,  474,  1920). 

If  one  product  of  the  reaction  is  insoluble,  the 
case  is  different,  and  diffusion  proceeds  only  from 
the  component  with  higher  osmotic  pressure  to  that 
with  lower  pressure.  If  the  solutions  are  isotonic,  an 
extremely  thin  layer  of  precipitate  is  formed,  and 
the  reaction  and  diffusion  do  not  proceed  any 
further.  This  has  been  demonstrated  by  Bechhold 
and  Ziegler  with  gelatin  containing  sodium  chloride 
covered  with  a  solution  containing  an  equivalent 
(and  isotonic)  solution  of  silver  nitrate.  A  barely 
visible  film  of  silver  chloride  is  formed  and  diffusion 
then  ceases.  They  showed  further  that  if  the  film 
of  gelatin  containing  AgCl  is  melted  and  allowed  to 
set  again,  the  reaction  proceeds  once  more,  a  result 
which  will  be  referred  to  later. 

The  rule  just  stated  that  diffusion  producing  an 
insoluble  reaction  product  proceeds  only  into  the 
component  of  lower  osmotic  pressure  was  first 
formulated  by  Pringsheim,  and  is  generally  called 
after  him.  Exceptions  occur  where  the  rate  of 
diffusion  is  very  markedly  different,  and  where  one 
component  has  a  strong  lyotropic  effect. 

Whether  the  reaction  proceeds  in  the  aqueous 
solution  or,  which  interests  us  more  at  the  moment, 
in  the  gel,  it  is  controlled  by  the  rate  of  diffusion, 
and  therefore,  generally  speaking,  proceeds  slowly, 
so  that  conditions  are  favourable  to  the  formation  of 
large  crystals  of  the  reaction  product.  The  author 
and  A.  Simon  were  able  to  produce  gold  crystals  up 
to  3  mm.  across  by  reducing  gold  chloride  in  silicic 
acid  gel  with  oxalic  acid  ;  Holmes  obtained  large 
tetrahedra  of  copper  by  reduction  of  copper  salts 
with  hydroxylamine,  also  in  silicic  acid  gel  (//.  Am. 
Chem.  Soc.,  40,  1187,  1918),  etc.  In  many  cases  the 
precipitate  tends  to  form  spherical  aggregates,  and 


SPHERICAL  AGGREGATES.  121 

often  perfect  spherolites  ;  earlier  investigators  like 
Harting  obtained  them  with  calcium  salts,  which 
are  of  particular  interest  in  view  of  their  deposition 
in  the  endo-  or  exo-skeletons  of  organisms.  Since 
then  very  many  compounds,  like  silver  chromate, 
barium  silicofluoride,  manganese  sulphide,  etc.,  have 
been  obtained  by  various  workers.  Fig.  16  is  a 
photomicrograph  between  crossed  nicols  of  sphere- 


FIG.    16.  —  SPHEROLITES    OF 
BASlF6  BETWEEN  CROSSED 

XlCOLS. 

lites  of  barium  silicofluoride,  showing  the  black  cross 
and  rings  of  several  orders. 

These  results  show  that  large  aggregates  of  very 
insoluble  substances  can  be  formed  by  reactions  in  a 
gel  in  comparatively  short  times  ;  they  thus  throw 
considerable  light  on  many  questions  of  physiology, 
pathology,  and  geology,  detailed  discussion  of  which 
is  beyond  the  scope  of  this  book. 

A  further  phenomenon  of  very  great  interest  was 
first  observed  by  Liesegang,  and  is  generally  called 


122      THE   LIESEGANG  PHENOMENON. 

after  him.  The  original  experiment  is  made  as 
follows  :  a  drop  of  silver  nitrate  solution  is  placed 
on  a  film  of  gelatin  gel  containing  a  little  potassium 
dichromate.  The  silver  salt  diffuses  into  the  gel, 
and  silver  dichromate  is  precipitated  which,  however, 
is  not  deposited  in  a  continuous  zone  round  the  drop, 


(b) 

FIG.    17. — LIESEGANG   STRATIFICATIONS.       (a)    CALCIUM    PHOS- 
PHATE IN  GELATIN,     (b)  LEAD  CHROMATE  IN  AGAR. 

but  in  concentric  rings  separated  by  apparently  clear 
intervals  which  increase  in  width  towards  the  peri- 
phery. A  number  of  other  reactions  have  since  been 
found  to  lead  to  the  same  result,  and  larger  volumes 
can  be  dealt  with  by  placing  the  gel  in  test  tubes  and 
pouring  the  aqueous  solution  on  top.  Fig.  17  shows 
such  stratifications  of  (a)  calcium  phosphate  in 
gelatin,  and  (b)  lead  chromate  in  agar  obtained  by 


THE   LIESEGANG   PHENOMENON.      123 

the  author.  In  the  former  case  a  solution  of  calcium 
chloride  diffused  into  the  gel  containing  trisodium 
phosphate,  in  the  latter  a  solution  of  sodium  dichro- 
mate  into  an  agar  gel  containing  lead  acetate. 

The  phenomenon  is  of  very  great  interest  for  the 
following  reason.  Very  many  banded  or  stratified 
structures  occur  in  both  organic  and  inorganic 
natural  products,  and  their  explanations  have  so  iar 
involved  assumptions  like  a  periodic  supply  of  one 
or  both  components,  or  a  periodicity  in  external 
conditions  like  temperature.  The  Liesegang  shows 
that  very  striking  periodic  structures  may  be  formed 
when  all  the  factors  involved  remain  constant,  or, 
at  any  rate,  do  not  undergo  periodic  variations 
synchronous  with  the  formation  of  strata. 

The  phenomenon  is  extraordinarily  sensitive,  in 
the  first  instance,  to  changes  in  the  nature  of  the  gel. 
Different  brands  of  gelatin  give  widely  differing 
results  with  a  given  reaction,  if  all  other  factor's 
are  kept  constant,  and  a  reaction  which  produces 
excellent  stratifications  in  gelatin  may  fail  to  do  so 
in  agar,  and  vice  versa.  Some  of  these  divergences 
are  undoubtedly  due  to  the  difference  in  the  protec- 
tive effect  of  the  various  gels,  which  tends  to  prevent 
the  formation  of  excessively  large  crystals  ;  the  effect 
is  very  great  in  gelatin,  slight  in  agar  and  almost 
negligible  in  silicic  acid  gels.  In  the  last  named  the 
formation  of  strata  is  therefore  comparatively  rare, 
while  that  of  large  crystals  is  fairly  common. 

There  is  at  present  no  generally  received  explana- 
tion of  this  very  striking  phenomenon.  A  theory 
was  propounded  by  Wilhelm  Ostwald  soon  after  its 
discovery,  based  on  the  assumption  that  the  reaction 
product  at  first  remains  in  supersaturated  solution, 
until  the  limit  of  "  metastability  "  is  reached,  when 
it  crystallizes  out.  The  reaction  product  existing 
in  a  certain  zone,  and  the  component  in  the  gel,  are 
thus  exhausted  round  the  precipitate,  and  a  further 


124      THE  LIESEGANG   PHENOMENON. 

stratum  can  form  only  when  the  metastable  limit 
has  again  been  reached  ;  owing  to  the  decreasing 
concentration  the  distances  between  successive  rings 
increase. 

Experiments  published  by  the  author  show  that 
stratifications  can  be  obtained  when  the  crystalline 
reaction  product  (PbI2)  is  present  throughout  the 
gel,  and  also  that  a  second  system  of  strata  can  be 
produced  in  a  gel  already  containing  one,  although 
in  both  cases  super  saturation  is  impossible. 

A  promising  theory  has  recently  been  advanced 
by  S.  C.  Bradford  and  supported  by  experimental 
evidence  (Bioch.  //.,  n,  14,  1918  ;  14,  29,  1920  ; 
14,  474,  1920).  He  suggests  that  the  reaction 
product  adsorbs  the  component  present  in  the  gel, 
so  that  a  zone  of  some  depth  below  the  last  layer  of 
precipitate  is  practically  free  from  the  latter,  or,  in 
other  words,  the  concentration  is  so  low  that  the 
solubility  product  of  the  precipitate  is  not  exceeded. 
The  component  in  the  aqueous  solution  must  there- 
fore diffuse  some  distance  again  until  this  condition 
is  satisfied,  i.e.,  strata  separated  by  clear  intervals 
will  be  formed.  As  adsorption  (see  Chapter  XXII.) 
depends  on  the  specific  surface  of  the  adsorbing 
material,  its  effect  will  not  be  marked  unless  the 
latter  reaches  a  certain  degree  of  dispersity,  and 
Bradford  has  shown  that,  by  increasing  the  latter 
systematically,  a  reaction  which  ordinarity  fails  to 
produce  stratifications  (e.g.,  silver  chromate  in  agar) 
can  be  made  to  yield  them  in  great  perfection. 
Further  critical  experiments  are  required  before  the 
theory  can  be  held  to  be  generally  established,  and 
the  whole  phenomenon  offers  a  very  attractive  field 
for  investigation. 


CHAPTER   XX. 

THE  reader  will  have  noticed  that  no  attempt  has. 
so  far,  been  made  to  fit  the  gels  into  the  classification 
of  disperse  systems  given  on  p.  8.  This  omission 
is  deliberate,  as  it  is  not  possible  to  assign  them  a 
place  d  priori,  and  the  only  feasible  procedure  is  to 
attempt  to  deduce  their  structure  from  the  properties 
which  have  been  briefly  described  in  the  preceding 
chapters. 

As  regards  the  dried  rigid  gels,  like  that  of  silicic 
acid,  the  case  is  comparatively  simple.  We  have 
seen  that  these  are  porous  masses  in  which  the  dimen- 
sions of  both  the  pores  and  of  the  walls  separating 
them  are  of  amicroscopic  order.  The  walls  must  be 
continuous  to  account  for  the  cohesion  of  the  gel, 
but  the  pores  must  also  communicate  very  completely 
in  view  of  the  ready  imbibition  of  liquids.  We 
assume,  therefore,  a  continuous  solid  and  a  continuous 
liquid  or  gaseous  phase — a  conception  which  offers 
no  particular  difficulty  ;  we  can  imagine  numerous 
macroscopic  models,  e.g.,  an  aggregate  of  polyhedral 
cells  with  perforated  cell  walls,  etc.  Substances  like 
pumice  stone  or  artificial  porous  materials,  the  pores 
of  which  can  also  be  readily  filled  writh  liquid,  are, 
no  doubt,  coarser  systems  of  the  same  kind. 

Gelatin  gels  hardened  by  various  agents,  e.g., 
alcohol,  behave  like  rigid  gels,  imbibe  organic 
liquids  and  on  drying  show  the  same  characteristic 
phenomena  as  silicic  acid  gel. 

The  difficulties  become  very  much  greater  when 
we  turn  to  the  fresh  silicic  acid  gel  and  to  the 
elastic  gels.  The  fundamental  questions  whether 


126     THEORIES   OF  GEL   STRUCTURE. 

these  gels  are  homogeneous  or  heterogeneous  systems, 
and,  in  the  latter  event,  what  are  the  geometrical 
form  and  the  state  of  aggregation  of  the  phases,  are 
answered  in  opposite  ways  by  different  investigators. 

The  view  that  gels  are  homogeneous  systems  is 
held — to  name  only  its  principal  supporters — by 
Pauli,  Katz,  Procter  and  Loeb.  Roughly  speaking, 
this  view  is  based  on  the  negative  ground  that  no 
assumptions  regarding  structure  are  necessary  to 
account  for  the  observed  properties  of  gels.  Thus 
Katz,  as  the  result  of  a  very  careful  and  exhaustive 
study  of  swelling,  arrives  at  the  conclusion  that  it 
does  not  differ  from  other  processes  of  solution,  so 
that,  e.g.,  a  gelatin  gel  is  simply  a  solution  of  water 
in  gelatin.  Procter  arrives  at  a  similar  result  largely 
through  the  investigation  of  the  swelling  of  gelatin 
in  acid,  the  "  uncomplicated  "  swelling  in  pure  water, 
as  well  as  similar  phenomena  in  non-aqueous  solvents, 
being  disregarded.  Pauli  distinguishes  between  gels 
possessing  a  structure  and  jellies  "  in  which  a 
demonstrable  structure  need  not  be  presupposed  "  : 
the  latter  are,  of  course,  the  systems  which  we  are 
now  considering.  Pauli  postulates  as  the  necessary 
condition  for  gel  formation  electric  neutrality  of  the 
particles  "  which  then  enter  into  the  mutual  relations 
characteristic  of  the  solid  state."  Considering  our 
very  incomplete  knowledge  of  the  solid  state,  this 
view  renders  nugatory  all  attempts  to  demonstrate 
or  elucidate  gel  structure  by  the  investigation  of, 
e.g.,  the  elastic  properties. 

The  assumption  of  heterogeneity  and  of  a  special 
structure  in  gels  is  older  than  the  opposite  theory, 
and  has  undergone  various  transformations  with  the 
development  of  the  science  and  with  the  improve- 
ments in  the  means  of  observation.  Its  chief 
support  at  present  is  the  fact  that  emulsoid  sols, 
which  set  to  gels,  are  generally  recognised  as  hetero- 
geneous and  disperse  systems,  and  there  is  no 


THEORIES   OF   GEL   STRUCTURE.      127 

reason  to  assume  that  these  characteristics  are  lost 
in  the  transformation  to  gel.  Wo.  Ostwald  main- 
tains that  the  state  of  aggregation  remains  the  same, 
i.e.,  that  gels  (the  fresh  gels  rich  in  dispersion  medium 
are  referred  to)  are  systems  of  two  liquid  phases, 
although  these  liquids  may  be  somewhat  anomalous. 
Such  systems  would  have  to  have  a  definite  geo- 
metrical configuration,  and  this  entails  some  difficul- 
ties in  accounting  for  their  elastic  properties,  which 
the  author  has  attempted  to  analyze  (Trans.  Faraday 
Soc.,  Vol.  XII.,  Part  i,  1916).  The  view  is  sup- 
ported, inter  alia,  by  the  continuity  of  the  trans- 
formation as  traced  by  viscosity  measurements. 
The  uncertainty  of  this  criterion  has  already  been 
pointed  out  (p.  82),  and  evidence  of  a  discontinuity 
is  provided  by  an  interesting  and  little  studied 
phenomenon,  the  shape  of  gas  bubbles  formed  in  the 
gel.  Such  bubbles,  which  can  be  produced  by  a 
variety  of  means,  are  always  lenticular  (E.  Hatschek, 
Koll.-Zeitschr.,  15,  226,  1914),  while  gas  bubbles  in  a 
liquid  at  rest — however  viscous — are,  of  course, 
spherical.  It  is  possible  to  produce  such  bubbles 
during  the  transformation,  and  to  note  an  abrupt 
change  from  the  spherical  to  the  lenticular  shape, 
which,  as  stated,  cannot  be  explained  by  a  mere 
continuous  increase  in  viscosity. 

The  earliest  attempts  to  ascertain  structure 
directly  by  microscopic  examination  are  due  to 
Butschli  (1892-1900),  who  investigated  silicic  acid 
and  elastic  gels  with  heroic  magnifications.  The 
influence  of  Quincke,  who  assumed  a  "  foam  cell  " 
structure  in  many  amorphous  substances,  manifests 
itself  in  these  investigations.  In  cases,  e.g.,  gelatin, 
where  the  microscope  did  not  reveal  a  structure,  a 
"  honeycomb "  configuration  showed  itself  after 
treatment  with  hardening  agents  such  as  chromic 
acid.  In  view  of  the  permeability  of  gels,  the  walls 
of  such  a  honeycomb,  i.e.,  polyhedral  structure,  must 


128  THE   MICELLAR  THEORY. 

either  offer  practically  no  resistance  to  the  passage 
of  solutes,  or  must  themselves  be  perforated.  The 
results  must  be  received  with  caution  on  two  distinct 
grounds  :  for  one  thing  the  interpretation  of  micro- 
scopic images  of  objects  of  this  magnitude  is  difficult, 
as  readers  who  may  have  examined  such  well-known 
test  objects  as  Pleurosigma  with  very  high  powers 
need  not  be  reminded.  For  another,  the  structure 
revealed  after  the  use  of  hardening  agents  may  not— 
as  emphasized  by  Pauli — have  been  a  pre-existing 
one  at  all.  Even  if  it  is,  it  is  not  the  ultimate 
structure,  which  from  ultra-microscopic  observations 
is  certainly  sub-  and  more  probably  amicroscopic. 

The  ultra-microscopic  study  of  gel  formation, 
combined  with  other  evidence  which  has  become 
available  with  the  progress  of  the  discipline,  has  led 
Zsigmondy  and  other  investigators  to  adopt  with 
some  modifications  a  theory  originally  propounded 
on  a  priori  grounds  by  Naegeli  to  account  for  the 
properties  of  elastic  gels.  The  element  of  structure, 
according  to  him,  is  a  molecular  aggregate,  generally 
anisotropic,  which  he  calls  a  "  micella  "  (diminutive 
of  mica,  a  crumb).  These  preserve  their  indi- 
viduality in  the  sol  and  in  the  gel  state  ;  the  latter 
is  caused  by  their  aggregation  or  linking  together. 
What  geometrical  form  this  linking  takes  is  uncer- 
tain, and  of  course  the  possibilities  are  very 
numerous  :  a  filamentous  structure  seems  as  probable 
as  any,  and  interesting  ultra-microscopic  observa- 
tions of  filament  formation  have  been  described,  e.g., 
by  McBain.  A  system  of  strands  or  an  open  net  or 
meshwork  would  account  for  the  easy  diffusion  into 
gels,  and  for  the  fact  that  liquid  containing  the 
colloid  can  be  pressed  out  of  them.  This  last 
phenomenon  has  been  studied  and  emphasized  by 
Hardy ;  whether  any  conclusion  regarding  the 
distribution  of  the  colloid  in  the  two  phases  con- 
stituting the  gel  can  be  drawn  from  the  respective 


GELS   OF  CRYSTALLINE  MATERIAL.     129 

concentrations  in  the  expressed  liquid  and  in  the 
residue  must,  however,  be  doubtful. 

Von  Weimarn,  who  obtained  transparent  gels  of 
a  number  of  substances  by  reactions  between  very 
concentrated  solutions  as  well  as  by  other  processes,  all 
of  which  in  principle  amount  to  producing  suddenly 
a  very  high  degree  of  supersaturation,  looks  on 
this  type  of  gels  as  composed  of  ultra-microscopic 
crystals.  Bradford  has  recently  (Bioch.  Journ.,  12, 
351,  1918)  attempted  to  extend  this  view  to  gels  like 
that  of  gelatin,  which  is  not  known  in  the  crystal- 
lized state  ;  the  evidence  adduced  by  him  will  require 
strengthening  before  it  can  be  generally  accepted. 

In  this  connection  it  is  of  interest  to  mention  the 
formation  of  very  typical  gels  from  a  material  which 
occurs  in  well-defined  crystals.  Several  instances 
are  known,  of  which  the  most  striking  is  camphoryl- 
phenylthiosemicarbazide,  which  has'  been  investi- 
gated by  the  author  (Koll.-Zeitschr.,  n,  158,  1912). 
The  substance  crystallizes  in  macroscopic  needles,  is 
slightly  soluble  in  cold  and  readily  in  boiling  organic 
solvents,  e.g.,  alcohol,  carbon  tetrachloride  or 
toluene.  If  a  5  per  cent,  solution  in  boiling  alcohol  is 
allowed  to  cool  slowly,  the  substance  crystallizes  in 
well-developed  crystals.  If  the  solution  is  cooled 
very  rapidly,  it  sets  to  a  clear  gel  with  a  slight 
bluish  opalescence  similar  to  that  of  silicic  acid  gel. 
On  standing,  crystal  rosettes  and  similar  aggregates 
begin  to  form  and  grow  with  a  rapidity  depending 
on  the  temperature,  liquid  is  segregated,  and  the 
crystals  finally  sink  to  the  bottom  in  clear,  cold- 
saturated  solution.  If  a  5  per  cent,  solution  of  the 
compound  in  boiling  carbon  tetrachloride  is  poured 
into  a  volume  of  warm  liquid  paraffin  sufficient  to 
make  the  final  concentration  about  i  in  350,  the 
mixture  on  cooling  sets  to  a  gel  as  clear  as  glass, 
which  persists  for  many  months,  although  groups 
of  small  crystals  may  become  visible. 

H.I.  9 


130    PROBABLE  GEL  STRUCTURES. 

In  this  instance  it  is  impossible  to  escape  the  con- 
clusion that  gel  formation  is  due  to  the  segregation 
of  ultra-microscopic  crystals  from  a  highly  super- 
saturated solution  ;  larger  crystals  then  grow  at  the 
expense  of  smaller  ones,  since  the  substance  is — 
though  very  slightly — soluble  in  the  cold  solvent. 
The  rate  of  growth  is  naturally  controlled  by  diffu- 
sion and  therefore  by  the  viscosity  of  the  medium. 
This  alone  is,  however,  not  sufficient  to  account  for 
the  formation  of  a  coherent  and  elastic  gel,  for  the 
following  reason.  If  we  assume  the  supersaturation 
to  be  equal  to  the  total  amount  of  substance  present, 
we  have  about  one  part  in  350,  and  it  is  easy  to 
produce  numerous  compounds  forming  needle-shaped 
crystals — e.g.,  calcium  sulphate,  calcium  carbonate 
as  arragonite,  etc. — in  the  same  supersaturation 
without  obtaining  anything  resembling  a  gel.  Some 
form  of  linking  or  continuous  structure  still  seems 
necessary  to  account  for  the  characteristic  properties 
of  the  gel. 

The  most  probable  view  is,  therefore,  that  in  all 
cases  of  gel  formation  there  is  a  linking  of  the 
elements  of  one  phase  into  some  continuous  structure, 
probably  largely  developed  in  one  dimension. 
Whether  this  consists  of  single  filaments,  of  strands 
or  fibrils  of  such,  or  of  a  more  complicated  network, 
or  finaFy  whether  in  some  cases  the  structure  consists 
of  crystals  twinned  or  otherwise  interlinked,  must  be 
left  open.  The  term  "  gel  "  is,  as  already  pointed 
out,  indefinite,  and  simply  designates  a  body  which, 
while  containing  a  large  amount  of  liquid,  shows  some 
of  the  properties  of  a  solid,  inasmuch  as  it  maintains 
its  shape  and  exhibits  elasticity.  The  range  of 
applicability  of  the  term  has,  indeed,  been  the  subject 
of  controversy  in  specific  cases,  such  as  soap.  It  seems 
advisable  to  use  it  exclusively  for  systems  which  do 
not  show  elements  of  structure  of  microscopic  size, 
though  possibly  this  distinction  is  also  arbitrary. 


CHAPTER  XXI. 

WE  have  already  had  occasion  to  emphasize  the 
importance  of  the  large  boundary  surface"  between 
the  phases  of  a  disperse  system,  inasmuch  as  it 
influences  the  electrical  properties,  and,  in  the  case 
of  two  liquid  phases,  has  an  obvious  bearing  on  the 
viscosity.  We  have  now  to  consider  a  very  large 
and  very  varied  class  of  phenomena  in  which  this 
surface  is  the  determining  factor,  all  of  which  may 
be  comprehensively  described  as  changes  of  concen- 
tration in  one  phase  at  its  boundary  surface  with 
another  phase.  A  study  of  these  changes  will 
enable  us  to  form  theoretical  views  of  some  funda- 
mental properties  of  colloidal  systems  of  which,  so 
far,  only  a  description  has  been  given. 

Many  instances  of  such  surface  phenomena  will  be 
familiar  to  the  reader,  either  from  text  books  or  from 
actual  experience.  One  is,  for  instance,  the  capacity 
of  charcoal  to  condense  large  volumes  of  gases,  of 
which  advantage  is  taken  for  obtaining  extremely 
high  vacua  and  for  removing  poison  gases  from  the 
atmosphere.  A  related  property  of  charcoal,  that 
of  taking  out  of  solutions  colouring  matter  or  the 
higher  alcohols  constituting  "  Fusel  oil,"  has  been 
known  since  the  end  of  the  eighteenth  century  and  is 
used  industrially  on  a  large  scale  ;  china  clay  and 
fuller's  earth  are  also  employed  for  purposes  of  the 
same  kind.  Another  phenomenon  of  the  same 
description  is  the  power  possessed  by  gels  like  gelatin 
or  isinglass  of  taking  down  turbidities  in  organic 
extracts.  An  instance  familiar  to  the  analyst  is 
the  well-known  fact  that  the  concentration  of  manv 


132     ADSORPTION  AND  SURFACE  ENERGY. 

solutions,  e.g.,  of  lead  salts,  is  perceptibly  reduced 
by  filtration  through  paper — that  is,  by  contact  with 
a  large  number  of  cellulose  fibres. 

This  list  could  be  extended  very  largely,  but  the 
instances  given  are  sufficient  to  show  that  in  all 
cases  the  effect  is  produced  at  a  boundary  surface 
of  very  considerable  extent.  What  occurs  is  that 
certain  substances  present  in  solution  are  concen- 
trated and  somehow  held  at  these  surfaces,  and  this 
change  in  concentration  is  now  generally  called 
adsorption. 

The  occurrence  of  these  changes  also  provides  us 
with  the  clue  to  their  investigation.  We  have  already, 
when  discussing  emulsions,  referred  to  the  fact  that 
the  surface  of  a  liquid  against  air  (more  strictly, 
against  its  own  vapour)  or  against  a  second  liquid 
is  in  tension,  known  as  surface  tension  in  the  former 
and  as  inter  facial  tension  in  the  latter  case.  This 
tension  is  an  extremely  well-defined  physical  con- 
stant, and  can  be  measured  by  a  number  of  methods. 
For  details  of  these  the  reader  must  be  referred  to 
the  text  books  of  physics  or  physical  chemistry  ; 
they  all  depend  on  the  tendency  of  the  tension  to 
reduce  the  surface  to  a  minimum  in  equilibrium  with 
the  other  forces — e.g.,  gravity — acting  on  the  body 
of  liquid  under  examination.  A  practical  acquaint- 
ance with  such  measurements,  even  when  slight,  will 
lead  rapidly  to  the  discovery  that  surface  tension  is 
very  easily  affected  by  exiguous  amounts  of  con- 
taminations, such  as  water  may  take  up  from  the  air 
in  a  few  minutes. 

As  it  takes  work  to  produce  or  enlarge  a  liquid 
surface,  it  is  the  seat  of  energy  which  is  measured  by 
the  product  of  surface  into  surface  tension  per  unit 
length  and  which,  of  course,  tends  to  become  a 
minimum.  We  already  know  one  wray  in  which  this 
may  be  accomplished,  viz.,  by  the  surface  becoming 
a  minimum  surface  ;  thus  a  drop  of  liquid  falling 


CHANGES  IN   SURFACE   ENERGY.     133 

freely  in  a  vacuum  or  suspended  in  another  liquid  of 
the  same  density  assumes  spherical  shape,  the  sphere 
having  the  minimum  surface  for  a  given  volume. 
Obviously  this  is  only  possible  where  both  phases 
are  easily  deformable,  i.e.,  when  both  are  -liquid,  or 
one  liquid  and  the  other  gas. 

The  question  now  arises  whether  such  tensions  and 
energies  also  exist  at  the  surfaces  gas-solid  and  liquid- 
solid,  especially  in  view  of  the  instances  quoted  above, 
all  of  which  refer  to  such  systems.  It  is  obvious 
that  the  immediate  methods  of  demonstrating  and 
measuring  such  tensions,  as  applied  to  easily  deformed 
phases,  fail,  since  the  surface  of  the  solid  is  not  easily 
altered  and  does  not  adjust  itself  to  the  tension.  We 
can,  therefore,  only  conclude  by  various  inferences, 
which  cannot  be  detailed  here,  that  such  tensions  do 
actually  exist,  and  the  evidence  on  this  point  is  quite 
conclusive.  The  question  then  arises  whether  the 
surface  energy  at  the  boundary  of  a  solid  can  be 
varied  and  reduced  to  a  minimum  by  some  other 
means.  Since  it  is  a  product  of  two  factors,  it  can 
of  course  be  reduced  by  reducing  either  ;  in  the  case 
of  a  solid  the  surface  is  constant  and  any  reduction 
which  may  occur  must  be  due  to  a  reduction  in  the 
surface  tension.  It  is  at  least  a  reasonable  assump- 
tion that  a  change  in  concentration  at  the  boundary 
surface  may  be  accompanied  by  such  a  reduction.  If 
this  assumption  is  correct,  such  a  change  ought  to 
take  place,  and  this  is  what  actually  happens. 

We  have  thus  come  to  the  conclusion  that,  if  in  a 
two-phase  system  a  change  in  the  concentration  of 
the  liquid  or  gaseous  phase  will  lead  to  a  decrease  of 
surface  energy,  this  change  will  take  place  :  to  prove 
this  view  we  have  to  investigate  whether  in  all  cases 
where  such  changes  appear  there  is  a  diminution  cf 
surface  tension.  A  change  of  concentration  can 
occur  in  the  gas  phase  if  it  is  compressed  at  the 
boundary  surface,  or  in  a  solution  if  the  solute  is 


134          THE   SURFACE  LIQUID-GAS. 

accumulated  at  the  boundary.  Now  we  know  that 
charcoal,  for  instance,  both  condenses  gases  on  its 
surface  and  removes  colouring  matter  from  solution, 
but,  as  already  pointed  out,  we  have  no  means  of 
measuring  the  surface  tension.  We  must  accordingly 
obtain  evidence  by  studying  similar  phenomena 
under  conditions  which  permit  such  measurements, 
and  this  is  possible  by  investigating  the  behaviour 
of  a  mercury  surface  against  gases.  The  subjoined 
table  gives  the  surface  tension  CT  of  mercury  against 
vacuum  (strictly  against  mercury  vapour  only)  and 
various  gases,  one  value  being  taken  immediately 
after  formation  of  the  surface,  the  other  one  hour 
later  :— 

Mercury  exposed  to  :          a-  fresh  surface.    <r  after  one  hour. 

Vacuum  (15°)      . .  . .  436  436 

Hydrogen  (21°)  ..  .  .  470  434 

Oxygen  (25°)       . .  ..478  432 

Nitrogen  (16°)     . .  . .  489  438 

Carbon  dioxide  (19°)  . .  480  436 

Dry  air  (17°)       .  .  .  .  476  429 

Moist  air  (17°)     .  .  . .  481  429 

It  will  be  seen  that  the  two  values  measured  in 
vacuo  are  the  same,  that  is,  no  change  has  taken 
place  after  one  hour.  In  all  the  gases,  however,  the 
final  value  of  the  surface  tension  is  considerably 
lower  than  the  initial  one,  and  this  change  is  accom- 
panied by  the  condensation  of  gas  on  the  mercury 
surface.  The  rate  of  change  is  characteristic  for 
each  gas,  and  curves  plotted  with  the  times  and 
tensions  as  co-ordinates  agree  very  well  with  the 
corresponding  time-pressure  curves  obtained  with 
solid  adsorbent  materials  exposed  to  the  same  gases. 
As  the  decrease  in  the  surface  tension  mercury-gas 
is  accompanied  by  compression  of  the  latter,  we 
must  at  once  conclude  that  the  surface  tension  of 


THE   SURFACE  LIQUID-GAS.  135 

mercury  should  be  lowered  by  rising  gas  pressure — a 
conclusion  which  has  been  experimentally  verified. 

The  case  discussed  furnishes  us  with  an  instance 
in  which  the  concentration— or,  in  other  words,  the 
pressure — of  gas  has  been  altered  at  the  surface  of  a 
liquid  with  a  reduction  of  surface  energy,  and  we 
conclude  that  the  same  conditions  hold  good  at  the 
boundary  solid-gas.  We  may  now  consider  an 
instance  at  the  boundary  liquid-gas  in  which  the 
change  of  concentration  occurs  in  the  liquid  instead 
of  the  gas  phase,  with  the  object  of  once  more 
verifying  that  it  is  accompanied  by  a  reduction  of 
the  surface  tension.  We  are  already  familiar  with 
one  simple  criterion  of  lowered  surface  tension  : 
froth  formation.  If  we  therefore  take  a  solution 
exhibiting  this  characteristic  and  produce  the 
largest  possible  surface  by  making  a  froth,  the 
latter  ought  to  contain  the  dissolved  substance  in 
greater  concentration  than  the  liquid  in  bulk.  This 
reasoning  has  also  been  verified  experimentally  by 
several  observers,  especially  by  Miss  Benson,  for 
mixtures  of  water  and  amyl  alcohol,  which  froth 
copiously.  Air  is  drawn  through  the  mixture  and 
carries  the  froth  formed  over  into  a  second  vessel  : 
this  froth  and  the  bulk  are  then  analyzed  separately. 
An  excess  of  about  5-5  per  cent,  of  alcohol  is  found  in 
the  froth,  which  result  confirms  our  reasoning. 

The  experiment  just  quoted  refers  again  only  to 
the  surface  liquid-gas,  but  it  can  be  shown,  e.g.,  with 
albumin  sols,  that  this  surface  and  the  interface 
liquid-liquid  behave  alike.  Such  sols  froth  copiously, 
and  it  was  shown  by  Ramsden  that  by  vigorous 
shaking  of  dilute  sols  practically  the  whole  of  the 
albumin  went  into  the  froth,  incidentally  becoming 
insoluble.  The  same  thing  occurs  at  the  interface 
liquid-liquid  ;  if  an  albumin  sol  is  shaken  vigorously 
with  an  organic  liquid,  e.g.,  chloroform  or  carbon 
tetrachloride,  the  latter  is  broken  up  into  small  drops 


136  GIBBS'S  THEOREM. 

which  do  not  coalesce  even  after  long  standing, 
owing  to  the  formation  of  an  albumin  membrane. 
If  the  excess  of  sol  is  poured  off  and  replaced  by 
water  the  drops  still  fail  to  coalesce,  the  membrane 
being  insoluble  just  like  the  albumin  concentrated  in 
froth,  i.e.,  at  the  surface  air-liquid. 

We  have  thus  some  evidence  to  support  the  view 
that  the  changes  of  concentration,  classed  together 
as  adsorption,  on  a  surface  are  due  to  the  tendency 
of  surface  energy  to  assume  a  minimum  value, 'and 
that  they  occur  if  an  increased  concentration  leads  to 
a  reduction  of  the  surface  tension.  Our  whole  know- 
ledge of  the  matter,  however,  is  not  exclusively  based 
on  such  reasoning,  which,  as  far  as  the  solid  surface 
is  concerned,  rests  on  the  uncertain  ground  of  analogy, 
but  the  principal  proposition  has  been  proved  by 
thermodynamical  methods  by  Willard  Gibbs.  He 
arrived  at  the  following  expression  : — 

jj  _          C    da 
"  RT  dC 

in  which  the  symbols  mean  : 

U  excess  of  substance  in  surface  layer. 

C  concentration  in  bulk  of  liquid, 

cr  surface  tension, 

R  the  gas  constant, 
*  T  the  absolute  temperature. 
The  formula  contains  the  differential  coefficient  of 
the  function  connecting  surface  tension  with  con- 
centration, which  is,  of  course,  positive  if  both 
change  in  the  same  sense,  and  negative  if  they  change 
in  opposite  senses.  This,  in  conjunction  with  the 
minus  sign  on  the  right  hand  of  the  equation,  shows 
at  once  that  there  will  be  a  negative  excess,  i.e.,  a 
reduced  concentration  in  the  surface,  if  the  surface 
tension  increases  with  increasing  concentration,  and 
a  positive  excess,  i.e.,  increased  concentration  in  the 
surface,  if  the  surface  tension  decreases  with  increas- 


GIBBS'S   THEOREM.  137 

ing  concentration.  The  latter  is  the  more  frequent 
case  and  agrees  with  the  instances  previously  given  ; 
the  former  has  also  been  observed  experimentally, 
and  is  known  as  negative  adsorption. 

Various  attempts  to  test  the  formula  quantita- 
tively have  been  made  ;  for  a  description  of  these 
and  a  full  discussion  of  the  difficulties  involved  the 
reader  is  referred  to  "  Surface  Tension  and  Surface 
Energy."  One  fundamental  difficulty  must,  how- 
ever, be  pointed  out  here.  The  coefficient  dajdC 
can,  of  course,  be  determined  only  from  measure- 
ments of  a-  at  different  concentrations,  and  these 
are  possible  only  at  the  interface  liquid-gas  or 
liquid-vapour.  The  assumption  that  da/dC  will  be 
the  same  at  the  interface  solid-liquid,  if  a  solid 
adsorbent  is  in  question,  is  not  susceptible  of  proof. 
As  a  matter  of  fact,  anomalous  cases  are  known,  in 
which  the  solute  raises  the  surface  tension  (da/dC  is 
positive),  and  is  yet  adsorbed  on  a  solid  adsorbent. 
Instances  are  sugar  on  charcoal  (G.  Wiegner,  KolL- 
Zeitschr.,  8,  126,  1911)  and  sodium  chloride  on 
colloidal  sulphur  (Sven  Oden,  loc.  cit.,  p.  42). 


CHAPTER  XXII. 

FURTHER  conclusions  can  be  drawn  from  the  Gibbs 
formula.  As  the  absolute  temperature  appears  in 
the  denominator,  the  excess  in  the  surface  or,  in 
other  words,  the  amount  adsorbed,  varies  inversely 
with  the  temperature  and  decreases  as  the  latter 
rises.  This  relation  holds  generally,  though  only 
qualitatively.  It  can  also  be  shown  that  a  small 
amount  of  dissolved  substance  can  lower  the  surface 
tension  greatly,  but  can  only  increase  it  slightly. 

This  statement,  though  perhaps  unexpected  at 
first  sight,  becomes  intelligible  when  we  remember 
that  surface  tension  manifests  itself  only  in  the  sur- 
face layer  and  depends  purely  on  the  composition 
of  the  latter.  If  a  dissolved  substance  increases 
the  surface  tension,  the  formula  tells  us  that  its 
concentration  in  the  surface  layer  is  less  than  in  the 
bulk  of  the  liquid,  and  its  effect  thus  tends  to 
counteract  itself.  On  the  other  hand,  if  it  reduces 
the  surface  tension,  it  accumulates  in  the  surface 
layer,  thus  enhancing  its  effect.  As  a  matter  of 
experience,  minute  amounts  of  accidental  contami- 
nations often  reduce  the  observed  values  of  surface 
tension  considerably,  while  increases  caused  by  such 
unintentional  admixtures  are  not  met  with. 

When  more  than  one  solute  is  present  the  process 
necessarily  becomes  complicated,  but  one  or  two 
points  may  be  discussed  briefly.  It  is,  generally 
speaking,  probable  that -the  various  substances  may 
not  be  adsorbed  to  the  same  extent,  in  which  case 
one  or  the  other  may  be  removed  selectively,  e.g., 
colouring  matter  from  organic  extracts.  Where  a 


ADSORPTION   AND   ADSORBENTS.     139 

compound  is  dissolved  in  a  dissociating  solvent  the 
ions  may  likewise"  not  be  adsorbed  equally,  and  the 
solution,  originally  neutral,  may  be  acid  or  alkaline 
after  adsorption  ;  this  has  also  been  observed  experi- 
mentally, e.g.,  by  van  Bemmelen  in  the  adsorption  of 
potassium  sulphate  by  gels,  when  the  remaining 
solution  was  found  acid,  the  K  ion  being  adsorbed 
to  a  greater  extent  than  the  S04  ion. 

If  an  adsorbent  is  placed  successively  into  two 
different  solutions  the  case  can  occur  that  the  second 
solute  is  adsorbed  more  strongly  than  the  first,  which 
it  will  then  displace  more  or  less  completely.  Thus, 
if  charcoal  is  shaken  with  a  dilute  solution  of  Fuchsin, 
the  dye  is  adsorbed  and  the  liquid  after  filtering  or 
centrifuging  off  the  charcoal  is  colourless.  If  the 
charcoal  is  now  placed  in  a  dilute  solution  of  saponin 
the  latter — which  lowers  surface  tension  greatly  and 
accordingly  causes  frothing  in  exiguous  concentra- 
tion— is  strongly  adsorbed  and  displaces  the  dye,  so 
that  the  solution  turns  red. 

It  is  obvious  that  the  amount  adsorbed,  other 
things  being  equal,  is  proportional  to  the  active 
surface.  All  substances  employed  as  adsorbents 
have  a  very  large  surface  per  unit  mass  ;  it  is 
unfortunately  impossible  to  measure  it,  and  com- 
parison between  two  adsorbents  leaves  open  the 
question  whether  differences  in  their  power  of 
adsorption  are  due  to  differences  in  specific  surface, 
or  to  specific  differences  in  the  materials,  or,  of 
course,  to  both.  Apart  from  the  materials  like 
charcoal,  fullers'  earth,  etc.,  which  are  familiar,  one 
which  has  recently  acquired  some  prominence 
deserves  mention.  This  is  the  modification  of 
aluminium  hydroxide,  called  by  Wislicenus,  who  first 
used  it  for  the  quantitative  adsorption  of  tannin, 
"  gewachsene  Tonerde,"  i.e.,  "  sprouted  alumina." 
It  is  obtained  by  allowing  aluminium  slightly  con- 
taminated with  mercury  to  oxidize  in  a  moist 


140    THE  ADSORPTION   EQUILIBRIUM. 

atmosphere  ;  the  microscopic  structure  singularly 
resembles  that  of  a  unicellular  fibre,  while  the  ulti- 
mate structure  is  probably  that  of  a  gel.  That  the 
adsorbent  effect  is  due  to  the  latter  structure  is 
shown  by  its  remaining  unaltered  when  the  substance 
is  dehydrated  at  red  heat. 

While  we  have  thus  a  number  of  qualitative  data 
regarding  the  phenomenon,  important  questions  are 
still  open  :  whether  it  proceeds  to  any  definite  end 
point  or  equilibrium,  what  time  is  required  to  reach 
such  equilibrium,  if  it  does  exist,  and  whether  the 
equilibrium  concentrations  can  be  formulated  mathe- 
matically. 

As  regards  the  first  point,  this  is  settled,  inasmuch 
as  a  definite  equilibrium  can  be  shown  to  exist  in 
cases  of  pure' adsorption,  i.e.,  when  chemical  action 
and  other  complications,  some  of  \vhich  are  still 
obscure,  are  excluded.  One  of  the  first  experiments 
dealing  with  this  aspect  of  the  problem  was  made  by 
Wilhelm  Ostwald,  who  placed  a  quantity  of  charcoal 
in  dilute  hydrochloric  acid,  and  after  a  certain  time 
determined  the  concentration  of  the  latter.  If,  then, 
a  portion  of  either  the  charcoal  or  of  the  acid  was 
removed  no  further  change  took  place,  which  tends 
to  show  that  the  two  concentrations,  in  the  surface 
layer  and  in  the  bulk  respectively,  are  independent 
of  the  masses,  a  necessary  condition  for  an  equili- 
brium. Further  decisive  experiments  were  carried 
out  by  Freundlich,  who  placed,  charcoal  in  solutions 
of  acetic  and  of  benzoic  acid  of  known  strengths  and 
determined  the  amounts  adsorbed.  The  same  quan- 
tities of  charcoal  were  then  placed  into  half  the 
volumes  of  acid  of  double  the  concentration  used  in 
the  first  experiments,  and  after  a  time  an  equal 
volume  of  solvent  was  added,  bringing  the  total 
volumes  to  those  used  in  the  first  instance.  If  there 
is  a  definite  equilibrium  between  the  adsorbed 
quantities  and,  the  concentrations  in  the  bulk  of  the 


THE  ADSORPTION   EQUILIBRIUM.     141 

solution,  the  final  concentrations  in  the  second  set 
of  .experiments  must  be  the  same  as  in  the  first,  and 
Freundlich  in  fact  found  this  to  be  the  case. 

As  regards  the  time  required  for  reaching  an 
equilibrium,  it  must  be  remembered  that  the  solute 
can  reach  the  adsorbing  surface  only  by  diffusion, 
which  therefore  controls  the  time,  provided  the  liquid 
and  adsorbent  are  at  rest.  Adsorption  experiments 
are  therefore  always  carried  out  with  agitation, 
generally  under  specified  uniform  conditions.  Equi- 
librium is  then  as  a  rule  reached  very  rapidly, 
but  numerous  cases  are  recorded  in  which  it  is  not 
final,  and  further  slow  withdrawal  of  solute  extends 
over  a  considerable  period.  A  striking  instance  is 
the  adsorption  of  iodine  by  charcoal  (J.  W.  McBain, 
Trans.  Far.  Soc.,  14,  Pt.  3,  1919),  which  continues 
slowly  for  many  months.  There  is  at  present  no 
definite  explanation  of  this  and '  similar  anomalies  ; 
in  some  instances  chemical  action,  although  at  first 
sight  highly  improbable,  has  been  demonstrated,  e.g., 
in  the  adsorption  of  permanganate  and  of  oxalic 
acid  by  charcoal.  In  this  connection  it  is  worth 
emphasizing  that  charcoal,  which  has  been  used  more 
largely  than  any  other  substance  in  the  study  of 
adsorption,  is  extremely  ill  defined  chemically  and, 
whatever  else  it  may  be,  is  certainly  not  simply 
"  carbon,"  although  it  is  frequently  so  described  in 
records  of  adsorption  experiments. 

The  relation  between  adsorbed  amount  and 
equilibrium  concentration  was  first  deduced  by 
Freundlich,  who  has  also  carried  out  a  large  amount 
of  varied  experimental  work  to  test  it.  If  we  call 
the  quantity  adsorbed  y,  the  quantity  of  adsorbent  m, 
and  the  end  or  equilibrium  concentration  in  the 
liquid  (after  adsorption)  C,  this  relation  takes  the 
following  form  :— 

y-=  aC, 

m 


142    THE  CONCENTRATION   FUNCTION. 

in  which  a  and  n  are  constants  depending  on  the 
nature  of  the  solutions  and  the  adsorbent.  The 
curve  corresponding  to  the  above  equation  was  called 
by  Freundlich  the  "  adsorption  isotherm/'  but  is 
now  more  generally  described  as  the  concentration 
function.  It  is  a  generalized  parabola,  and  for  n  =  2 
it  becomes  the  ordinary  conic  parabola. 

It  may  be  pointed  out  here  that  the  forrmTa  is  still 
very  frequently,  but  quite  incorrectly,  spoken  of  as 
an  "  exponential  "  one.  An  exponential  expression 
is  one  containing  one  of  the  variables — in  the  present 
case  these  are  y  and  C — as  exponent,  whereas  the 
exponent  of  the  concentration  function  is  a  constant. 
It  is  an  interesting  fact  that  this  constant  varies 
within  comparatively  narrow  limits  for  the  most 
widely  different  solutes,  viz.,  roughly  speaking, 
between  n  ==  2  and  n  =  10. 

The  principal  deduction  from  the  equation  is 
obvious  :  the  amount  adsorbed,  other  things  being 
equal,  increases  much  more  slowly  than  the  concen- 
tration or,  in  other  words,  is  proportionately  greatest 
in  dilute  solution. 

The  concentration  function  has  been  tested,  and 
the  exponents  i/n  determined,  for  many  solutes  and 
solvents.  The  table  below  is  due  to  Freundlich  :— 


Adsorbent. 

Solvent. 

Solute. 

i 

n. 

Blood  charcoal 

Water 

Formic  acid 

0-451 

Acetic  acid 

. 

0-425 

Benzoic  acid 

•    i     0-338 

Picric  acid 

0-240 

Chlorine  .  . 

, 

0-297 

Bromine  .  . 

•    |     0-340 

Benzene 

Benzoic  acid 

.    |     0-416 

Picric  acid 

.    ;     0-302 

Water 

Patent  Blue 

0-190 

Wool 

Patent  Blue 

0-159 

Silk 

" 

Patent  Blue 

0-163 

THE  CONCENTRATION   FUNCTION.    143 

The  table  gives  a  good  idea  of  the  limits  between 
which  the  exponent  varies,  and  a  further  point  of 
interest  is  raised  by  the  behaviour  of  the  same  solute 
— patent  blue  in  water — towards  three  adsorbents 
as  different  as  charcoal,  wool  and  silk.  The  value  of 
the  exponent  does  not  differ  greatly  in  the  three 
cases,  and  it  has  been  observed  with  many  solutes 
that  the  influence  of  the  adsorbent  is  slight  compared 
with  that  of  other  factors.  It  must,  however,  be 
remembered  that  the  comparison  between  various 
adsorbents  is  based  on  equal  weights  in  equal  volumes 
of  solution,  while  the  important  factor  is  the  surface, 
which  we  have  no  means  of  determining  and  com- 
paring, at  least  with  any  approach  to  accuracy. 
Specific  effects  are,  of  course,  possible  ;  thus  Michaelis 
and  Rona  (Koll.-Zeitschr.,  25,  225,  1919)  conclude 
from  experiments  with  solutions  of  acetone  and 
some  higher  alcohols,  with  various  adsorbents,  that 
the  action  of  charcoal  is  specific  and  not  expli- 
cable merely  by  any  probable  difference  in  active 
surface  compared  with  the  other  materials  investi- 
gated. 

It  is  fairly  well  established  that  the  order  in  which 
various  solutes  are  adsorbed  is  the  same  for  different 
adsorbents  ;  if  a  substance  A  is  more  strongly 
adsorbed  than  another  B,  and  the  latter  more  than 
C,  by  charcoal,  the  same  order  will  hold  good  for 
other  adsorbing  materials,  although  the  numerical 
ratios  may  differ. 

Another  question  of  importance  has  not  been 
touched  on  so  far — that  is,  the  effect  of  the  sol- 
vent in  cases  where  a  substance  is  soluble  in  more 
than  one  liquid.  It  is  well  known,  even  from 
general  experience,  that  the  same  substance  is  not 
adsorbed  equally  out  of  solutions  in  different  sol- 
vents, and  that  adsorption  is  much  slighter  in 
organic  solvents  than  in  aqueous  solution.  Thus 
FreundUch  gives  the  following  figures  for^  the 


144 


ADSORPTION   CURVES. 


adsorption  of  benzoic  acid  out  of  solutions  of  equal 
strength  in  :— 

Water  . .  . .  . .  4-27 

Benzene  . .  . .  .  .  0*55 

Ether  . .  . .  . .  030 

Acetone  . .  . .  . .  0-30 

The  difference  is  probably  explained  by  the  high 


FIG.   18. — TYPICAL  ADSORPTION  CURVES. 

surface  tension  of  water  (73  dynes/cm.)  compared 
with  that  of  organic  liquids,  e.g.,  benzene  28'8, 
acetone  23  and  ether  16*5  dynes/cm.  The  lowering 
of  surface  tension  caused  by  solutes  is  correspondingly 
greater  in  water,  and  therefore  also  the  adsorption 
from  aqueous  solutions.  This  difference  is  used 
practically  [for  removing  or  recovering  from  an 
adsorbent  substance  taken  up  by  it  out  of  an  aqueous 
solution.  If,  for  instance,  a  dilute  solution  of  a  dye/ 


ANOMALOUS  ADSORPTION. 


145 


like  crystal  violet,  is  shaken  with  sufficient  charcoal, 
the  dye  may  be  removed  completely  by  the  latter. 
If  the  water  is  now  replaced  by  alcohol,  the  adsorption 
from  which  is  much  lower,  the  surface  concentration 
of  the  dye  on  the  charcoal  is  in  excess  of  that  which 
would  establish  equilibrium  and  a  great  portion  of 
it  goes  into  solution  in  the  alcohol.  It  is  hardly 
necessary  to  point  out  that  this  effect  is  quite  distinct 
from  the  displacement 
by  another  solute  de- 
scribed on  p.  139. 

Two  typical  adsorp- 
tion curves  are  shown 
in  Fig.  18.  They  are 
due  to  Freundlich  and 
strikingly  illustrate  the 
parabolic  shape  cha- 
racteristic  of  the 
normal,  uncomplicated 
phenomenon.  Similar 
curves  which  can  be 
represented  fairly  ac- 
curately by  equations 

of  the  form  given   on   FTG.  I9._ANoMALous  ADSORPTION 
p.  141  have    been    ob-          OF  NIGHT  BLUE  BY  COTTON. 
tained   by   Freundlich 

and  other  investigators  for  numerous  other  non- 
electrolytes.  It  must,  however,  be  mentioned  that 
a  number  of  cases  are  known  in  which  the  concen- 
tration function  appears  to  be  of  a  fundamentally 
different  type,  and  more  particularly  some  in  which 
adsorption  reaches  a  maximum  at  definite  concentrations 
and  decreases  with  a  further  increase  in  concentration. 
An  interesting  instance  is  afforded  by  the  adsorption 
of  Night  Blue  on  various  adsorbents,  studied  by 
Biltz  and  Steiner  (Koll.-Zeitschr.,  7,  113,  1910). 
Fig.  19  shows  the  curves  obtained  with  cotton  as 
adsorbent,  (i)  being  that  for  the  technical  dye  at 


146    ADSORPTION   OF   ELECTROLYTES. 

room  temperature  ;  (2)  dialysed  dye  at  room  tem- 
perature ;  and  (3)  dialysed  dye  at  boiling  point. 
The  curves  agree  in  type  and  show  a  well-marked 
maximum.  As  Night  Blue  is  a  positively  charged 
colloid,  while  cotton  in  the  solutions  used  is  pro- 
bably negative,  adsorption  may  be  complicated  by 
electrical  factors,  though  these  do  not  very  readily 
account  for  the  maximum. 

Such  complications  are  even  more  marked  in  the 
case  of  strong  electrolytes,  and  at  present  very  little 
that  is  definite  can  be  stated  regarding  their  adsorp- 
tion. In  many  instances,  e.g.,  alkaline  nitrates  and 
sulphates,  adsorption  is  extremely  slight  ;  that  of 
the  chlorides  is  negative  according  to  Lagergren, 
while  Sven  Oden,  by  various  inferences,  reaches  the 
conclusion  that  sodium  chloride  is  adsorbed  positively, 
and  to  a  considerable  extent,  by  colloidal  sulphur 
(loc.  cit.,  p.  42).  Salts  of  heavy  and  noble  metals 
(lead,  gold)  can  be  removed  almost  completely  from 
dilute  solutions  by  charcoal,  but  the  liquid  is  left 
strongly  acid.  Various  explanations  are  possible  : 
from  gold  salt  solutions  the  hydroxide  formed  by 
hydrolytic  dissociation  may  be  adsorbed,  leaving 
the  free  acid  in  the  solution.  Although  the  first  step 
in  these  processes  may  be  a  pure  adsorption,  the  final 
condition  is  not  a  reversible  equilibrium.  The 
extreme  complications  which  may  arise  are  well 
illustrated  in  a  series  of  experiments  by  Oryng 
(Koll.-Zeitschr.,  n,  169,  1912)  on  the  adsorption  of 
potassium  permanganate  by  charcoal.  An  apparent 
equilibrium  is  reached  rapidly,  and  the  solution  is 
then  strongly  alkaline,  i.e.,  K*  is  not  adsorbed  in 
equivalent  amount  with  Mn04'.  On  further  standing 
or  shaking,  however,  both  Mn04'  and  K'  continue  to 
be  withdrawn  from  the  solution.  Experiments  were 
made  to  explain  this  secondary  phenomenon  and 
seemed  to  indicate  a  chemical  reaction  between 
MnO4/  and  the  charcoal,  oxalic  acid  and  MnO2  being 


ADSORPTION   OF   ELECTROLYTES.     147 

formed.  The  continued  disappearance  of  K '  is  most 
probably  due  to  adsorption  by  the  MnO2  thus 
formed. 

It  has  been  necessary  to  point  out  thus  briefly  the 
unsatisfactory  state  of  our  knowledge  regarding 
adsorption  from  electrolyte  solutions  because  the 
latter  are  of  particular  importance  to  the  colloidal 
chemist,  and  because,  as  we  shall  see  later  on, 
conclusions  drawn  from  the  uncomplicated  concen- 
tration function  have  been  put  forward  to  explain 
phenomena  observed  in  solutions  of  electrolytes. 


CHAPTER   XXIII. 

THE  different  degree  of  adsorption  of  two  or  more 
substances  present  in  a  solution  can  be  strikingly 
demonstrated,  and  can  be  utilized  for  proving  their 
presence  in  extremely  minute  quantities,  by  allowing 
the  solution  to  rise  by  capillarity  in  some  porous 
adsorbent  material,  e.g.,  in  strips  of  white  filter  paper. 
While  this  takes  place  the  dissolved  substances  are 
adsorbed  by  the  fibre,  so  that  beyond  a  certain 
height  the  liquid  in  the  paper  consists  of  pure 
solvent  only.  Different  solutes,  generally  speaking, 
rise  to  different  heights  and  can  be  identified  by 
their  colour  or,  if  colourless,  by  appropriate  reactions. 
The  process  can  be  demonstrated,  for  instance,  with 
a  dilute  solution  of  turmeric  and  picric  acid,  which  is 
allowed  .to  rise  in  a  strip  of  white  filter  paper  about 
12  ins.  long.  The  paper  is  stained  a  uniform  pale 
yellow,  but  if  it  is  then  exposed  to  ammonia  only 
the  lower  portion  turns  brown,  showing  that  the 
turmeric  pigment  has  not  risen  as  far  as  the  picric 
acid.  The  method,  which  deserves  to  be  more 
widely  known  than  appears  to  be  the  case,  may  be 
used  for  showing  the  presence  of  colouring  matter  or 
preservatives  in  articles  of  consumption,  and  for 
many  similar  purposes  even  \vhen  very  small  quan- 
tities only  are  available.  It  has  been  developed — 
under  the  title  of  "  capillary  analysis  "  —principally 
by  F.  Goppelsroeder,  who  has  demonstrated  its 
extreme  sensitiveness  in  favourable  cases. 

A  promising  modification  of  the  method  has 
recently  been  suggested  by  W.  Kraus  (Koll.-Zeitschr., 
28,  161,  1921),  which  is  based  on  the  observation  that 


CRITERIA  OF  ADSORPTION.          149 

when  a  porous  body  impregnated  with  a  solution  is 
allowed  to  dry,  the  solutes  are  concentrated  near  the 
surface.  Strips  of  filter  paper,  saturated  with  the 
solution  to  be  examined,  are  placed  between  glass 
plates,  one  end  being  allowed  to  protrude  and  to 
dry.  The  solutes  wander  towards  the  drying  end 
and  separation  takes  place,  those  most  strongly 
adsorbed  being  left  behind,  while  the  constituents 
which  are  adsorbed  slightly  are  concentrated  in  or 
near  the  exposed  end  of  the  strip. 

It  is  obvious  that  in  all  instances  in  which  reactions 
take  place  in  the  presence,  or  lead  to  formation,  of 
finely  divided  solid  matter,  adsorption  is  possible, 
and  may  account  for  changes  in  concentration  and 
losses.  It  offers  thus  a  somewhat  easy  explanation 
of  many  such  phenomena,  which,  however,  should 
not  be  taken  as  established  without  investigation. 
In  the  first  instance  it  is  necessary  to  demonstrate 
that  a  real  reversible  equilibrium  has  been  attained, 
since  the  adsorption  equation  holds  only  for  that 
state.  Measurements  at  different  concentrations 
must  then  be  made,  and  if  the  results  plotted  in  the 
usual  manner  lead  to  curves  of  the  parabolic  type 
shown  in  Fig.  18,  the  evidence  of  adsorption  may 
be  considered  strong,  though  not  absolutely  con- 
clusive, as  the  parabolic  type  of  curve  is  not  uncom- 
mon. If  the  results  show  a  marked  divergence  from 
the  type,  further  investigation  is  certainly  called 
for.  Adsorption,  if  it  takes  place  at  all,  may  then 
be  accompanied  or  followed  by  some  other  process, 
such  as  chemical  combination  or  penetration  into 
the  interior  of  the  adsorbent. 

A  process  which,  in  many  ways,  may  be  looked 
upon  as  the  converse  of  adsorption,  is  the  extraction 
of  a  substance  containing  an  admixture  with  a  solvent 
in  which  the  latter  only  is  soluble.  It  is  evident 
that  if  there  is  simple  mechanical  mixture,  and  if  a 
sufficient  quantity  of  solvent  is  employed,  the  whole 


150     EXTRACTION  OF  ADSORBED  MATTER. 

of  the  soluble  matter  will  be  extracted  by  this  first 
lot  of  solvent.  It  is  equally  obvious  that  this  cannot 
happen  if  the  second  substance  is  adsorbed  by  the 
first ;  in  that  event  the  first  lot  of  solvent  will, 
indeed,  remove  a  large  fraction  of  the  soluble  matter, 
but  as  much  of  it  as  establishes  equilibrium  under 
the  given  conditions  will  be  retained.  A  second  lot 
will  again  remove  a — much  smaller — quantity,  and 
so  on.  If  a  curve  is  plotted  with  the — preferably 
equal — volumes  of  solvent  as  abscissae,  and  the 
amounts  of  soluble  matter  still  retained  by  the 
insoluble  phase  as  ordinates,  curves  of  a  hyperbolic 
type  are  obtained,  and  it  is  quite  easy  to  construct 
from  these  the  inverse,  i.e.,  concentration-adsorption 
function,  as  will  be  clear  from  an  actual  example. 
Rubber,  as  is  well  known,  contains  varying  amounts 
of  "  resin,"  i.e.,  of  substances  soluble  in  acetone,  and 
the  results  of  extraction  with  equal  successive 
amounts  of  that  solvent  are  shown  in  Fig.  20,  taken 
from  an  investigation  by  D.  Spence  and  J.  H.  Scott, 
published  in  the  Kolloid-Zeitschrift.  The  amounts  of 
resin  still  retained  after  each  extraction  are  plotted 
as  ordinates  in  full  line  at  equal  distances  apart.  It 
will  be  noticed  at  once  that  the  first  lot  of  solvent 
extracts  a  very  large  proportion  of  the  total  resin 
content,  as  shown  by  the  length  ab  on  the  ordinate 
ac.  Similarly,  the  portion  extracted  by  the  second 
lot  of  solvent  is  given  by  the — much  smaller — length 
a'b'  on  a'c'.  The  curve  resembles  an  hyperbola,  and 
it  can  easily  be  shown  graphically  that  the  whole 
process  is  an  inverted  adsorption.  If  we  consider 
the  two  portions  of  the  ordinate  ac,  we  see  that  ab 
has  gone  into  solution,  while  be  is  retained  by  the 
rubber.  The  latter,  therefore,  represents  the  amount 
adsorbed,  which  is  in  equilibrium  with  the  concen- 
tration produced  by  dissolving  the  quantity  ab.  If, 
accordingly,  we  plot  the  lengths  ab,  a'b'  as  abscissae, 
and  the  lengths  be,  b'c',  as  ordinates,  we  obtain  a 


EXTRACTION   CURVE. 


curve  which  must  have  the  character  of  the  adsorp- 
tion curve  if  the  extracted  matter  was  really 
adsorbed  on  the  insoluble  portion.  The  curve  is 
plotted  in  dotted  line,  and  is  of  the  familiar  parabolic 
type. 

(In  the  actual  plotting  of  the  dotted  curve  the 


FIG.  20. — EXTRACTION  OF  ADSORBED  SUBSTANCE. 

ordinates  have    been    doubled    to    obtain  a  larger 
scale.) 

We  have  so  far  considered  only  phenomena  in 
which  the  change  in  surface  energy  has  been  held  to 
be  the  determining  factor,  and  have  disregarded  the 
fact,  with  which  we  are  already  familiar,  that 
boundary  surfaces  are  generally  the  seats  of  electric 


152  ELECTRIC  ADSORPTION. 

charges.  It  is  more  than  probable  that  these  may 
affect  adsorption,  and  there  are  some  striking 
phenomena  in  which  the  electric  factors  play  the 
most  important  or,  indeed,  an  exclusive  part.  If,  for 
instance,  a  dialysed  sol  of  ferric  hydroxide  is  passed 
through  a  column  of  carefully  purified  and  ignited 
quartz  sand,  the  ferric  hydroxide  is  completely 
retained  and  only  clear  water  leaves  the  end  of  the 
column  for  a  time.  The  same  thing  occurs  with  a 
sol  of  Night  Blue,  as  has  been  shown  by  Dreaper  and 
Davis.  In  both  cases  the  sand  is  capable  of  retaining 
only  a  definite  quantity  of  disperse  phase,  so  that 
the  process  is  obviously  not  a  filtration.  Both  ferric 
hydroxide  and  Night  Blue  belong  to  the  not  very 
numerous  class  of  positive  colloids,  while  silica 
assumes  a  negative  charge  in  contact  with  water. 
It  is  therefore  reasonable  to  assume  that  the  positive 
colloidal  particles  are  discharged  and  retained  by 
the  negatively  charged  sand  grains.  This  view  is 
borne  out  by  the  fact  that  acid  sols  of  ferric  hydroxide, 
in  which  the  H  ion  concentration  is  sufficient  to 
neutralize  or  reverse  the  charge  on  the  sand,  pass 
through  practically  unchanged.  Night  Blue  is 
retained  with  such  tenacity  even  by  smooth  glass 
surfaces  that  vessels  which  have  contained  the  sol 
cannot  be  washed  clean  with  water  alone. 

The  vast  general  importance  of  adsorption  hardly 
needs  to  be  emphasized.  Its  close  connection  with, 
and  special  importance  in,  the  stud}/  of  colloids  is 
also  obvious  ;  since  all  the  systems  dealt  with  under 
this  head  possess  very  large  interfaces,  adsorption 
is  an  essential,  if  sometimes  very  obscure,  factor  of 
the  whole  complex  of  phenomena  to  be  observed. 
Thus  adsorption  undoubtedly  takes  place,  not  only 
in  gels,  but  also  on  the  surface  of  the  disperse  phase 
in  sols.  This  has  been  proved  directly,  by  means  of 
conductivity  measurements,  by  Wolfgang  Ostwald 
and  by  several  other  observers.  Many  authorities 


ADSORPTION   OF  IONS.  153 

even  hold  that  the  electric  charge  on  the  particles 
is  due  to  absorbed  ions,  and  that  its  neutralization 
or  reversal,  with  coagulation,  by  electrolytes  belongs 
to  the  same  category.  This  aspect  of  the  subject 
will  receive  detailed  consideration  in  the  next  chapter. 


CHAPTER   XXIV. 

WE  are  now  in  a  position  to  survey  very  briefly 
the  theories  which  have  so  far  been  advanced  to 
account  for  the  electric  charges  in  disperse  systems, 
which  we  have  found  throughout  to  be  a  factor  of 
fundamental  importance.  At  the  outset  it  may  be 
useful  to  remind  the  reader  that,  while  theory  is  still 
highly  controversial,  the  facts  are  securely  established 
for  a  large  number  of  systems  and  can  easily  be 
ascertained  for  any  fresh  case  by  recognized  methods, 
such  as  cataphoresis,  etc. 

The  formal  mathematical  treatment  of  phenomena 
like  electric  osmosis  and  cataphoresis  rests  on  the 
assumption  that  at  the  boundary  surface  of  the  two 
phases  there  exists  an  electrical  double  layer,  a  con- 
cept and  term  introduced  by  Helmholtz.  This  means 
two  layers  or  surfaces  of  opposite  sign  and  a  definite 
difference  of  potential  separated  by  a  small  distance. 
The  mathematical  treatment  further  implies  that 
the  whole  of  the  double  layer  lies  in  the  liquid  phase,  i.e., 
the  one  charge  in  the  layer  of  liquid  immediately 
adjacent  to  the  solid  phase,  and  the  opposite  charge 
at  a  small  distance  away  from  the  interface.  In  an 
electric  field  the  two  faces  of  the  double  layer  are 
displaced  towards  each  other  with  the  result  that 
whichever  phase  is  free  to  move  travels  towards  the 
pole  carrying  the  opposite  sign. 

This  treatment  has  been  developed  by  Helmholtz, 
Lamb  and  v.  Smoluchowski,  and  leads  to  quantitative 
results,  but  makes  no  assumptions  about  the  origin 
of  the  double  layer  itself.  A  fortiori  it  does  not 
explain  the  conditions  which  we  have  described, 


THEORIES   OF  ELECTRIC   CHARGE.   155 

such  as  the  large  number  of  suspensoids  in  which  the 
disperse  particles  are  negatively  charged,  the  positive 
charge  in  a  fairly  limited  class  like  the  hydroxide  sols, 
and  the  reversal  of  the  charge  by  appropriate  ions. 
An  empirical  suggestion  to  account  for  the  sign  of 
the  charge  has  been  put  forward  by  Coehn,  according 
to  which,  of  two  phases  in  contact,  the  one  with  the 
higher  dielectric  constant  assumes  a  positive  charge. 
The  dielectric  constant  of  water  is  exceptionally 
high  (water  Si,  ethyl  alcohol  26,  benzene  2-3,  etc.), 
so  that  the  majority  of  substances  should  be  nega- 
tively charged  in  contact  with  it.  But  apart  from 
not  in  any  way  explaining  the  origin  of  the  charge 
the  rule  fails  to  account  even  for  the  sign  in  such  cases 
as  the  hydroxides.  Nothing  appears  to  be  known 
regarding  their  dielectric  constants,  but  in  the 
absence  of  such  knowledge  it  is  difficult  to  believe 
that  stannic  acid,  which  is  negative,  should  have  a 
lower,  and  ferric  or  aluminium  hydroxide,  which  are 
positive,  a  higher  dielectric  constant  than  water. 

A  further  difficulty  is  the  fact — of  great  importance 
in  other  respects,  although  frequently  overlooked — 
that  the  same  substance  may  have  either  -a  negative 
or  a  positive  charge,  according  to  the  method  by  which 
it  is  produced  as  disperse  phase.  Lottermoser's 
method  of  preparing  sols  of  the  silver  haloids  has 
already  been  mentioned  briefly  (p.  43).  According 
to  the  order  of  procedure  these  sols  may  be  negative 
or  positive  :  if  the  halogen  ion  is  in  excess  throughout, 
the  particles  of  silver  haloid  are  negatively  charged, 
while  the  opposite  result  is  obtained  by  keeping  the 
silver  ion  in  excess  throughout.  Other  cases  of  the 
kind  are  known  :  thus,  although  all  the  gold  sols 
described  in  the  literature  are  negatively  charged. 
Morris- Airey  and  Long  have  prepared  a  positive  sol, 
and  Powis  has  described  a  negative  ferric  hydroxide 
sol.  The  instances  given,  although  not  numerous, 
are  at  any  rate  sufficient  to  show  that  sign  of  the 


156  ADSORPTION   OF  IONS. 

charge  is  not   a  specific  characteristic  of  a  given 
substance. 

In  view  of  this  conclusion  it  is  natural  to  look  for 
the  cause  of  the  charge  in  the  aqueous  phase,  or 
more  precisely  in  the  ions  present  in  it.  As  has 
already  been  pointed  out  several  times,  the  aqueous 
phase  in  all  ordinary  sols  contains  electrolytes, 
which  are  quite  obviously  connected  with  the  forma- 
tion and  stability  of  the  sols.  One  theory  of  the 
electric  charge — put  forward  by  various  investigators, 
and  elaborated  by  Freundlich — accordingly  ascribes 
it  to  the  adsorption  of  ions  on  the  particles  of  disperse 
phase.  Among  the  ions  known  to  be  easily  adsorbed 
OH'  and  H'  are  pre-eminent.  Thus,  to  take  one  or 
two  concrete  examples,  the  reduction  of  gold  is  very 
generally  carried  out  in  alkaline  solution  ;  it  has, 
indeed,  been  suggested  (Pauli)  that  stable  gold  sols 
cannot  be  obtained  except  in  alkaline  media.  OH' 
is  accordingly  present  and,  being  an  anion,  imparts 
the  negative  charge  to  the  particles.  On  the  other 
hand,  in  the  usual  methods  of  preparing  ferric 
hydroxide  sol  the  medium  is  acid,  i.e.,  contains  H', 
which  is  adsorbed  and  imparts  the  positive  charge 
to  the  disperse  phase.  If,  however,  ferric  hydroxide 
is  made  by  Powis's  method — adding  ferric  chloric  to 
an  excess  of  caustic  alkali — OH'  is  present  and, 
being  adsorbed,  leads  to  the  production  of  a  negative 
sol. 

The  adsorbed  ions  must  necessarily  be  assumed  to 
be  concentrated  in  the  layer  of  liquid  immediately 
adhering  to  the  surface  of  the  disperse  phase,  and  to 
be  surrounded  by  a  layer  containing  an  equivalent 
amount  of  ions  of  the  opposite  sign.  We  therefore 
have  a  double  layer  which,  as  postulated  by  the 
Helmholtz  theory,  lies  entirely  in  the  liquid. 

According  to  Freundlich,  the  neutralization  of  the 
charge  is  also  produced  by  adsorption,  viz.,  of  the 
ion  with  a  sign  opposite  to  that  which  gives  the 


THEORY   OF  COAGULATION.          157 

particle  its  charge.  The  most  immediately  striking 
feature  of  electrolyte  coagulation,  the  effect  of 
valency,  he  explains  by  the  following  considera- 
tions :— 

To  neutralize  a  given  charge,  equivalent  amounts 
of  ions  of  different  valency  will  have  to  be  adsorbed, 
viz.,  the  same  effect  will  be  produced  by  one  uni- 
valent  ion,  1/2  bivalent  and  1/3  trivalent  ion,  or  the 
ratio  of  the  amounts  to  be  adsorbed  will  be  6  :  3  :  2. 
Assuming  the  ions  to  be  adsorbed  equally,  the  same 
concentration  function  will  represent  them,  so  that 
the  concentrations  corresponding  to  these  adsorbed 
amounts  will  be  the  abscissae,  the  ordinates  of  which 
are  in  the  above  ratio.  As  we  have  seen  (p.  142), 
the  adsorbed  amounts  increase  much  more  slowly 
than  the  equilibrium  concentrations  :  if  we  assume 
the  exponent  in  the  adsorption  equation  to  have  the 
value  1/3,  the  end  concentrations  giving  adsorbed 
amounts  in  the  above  ratio  would  be  216  (univalent)  : 
27  (bivalent)  :  8  (trivalent). 

How  far  the  simple  adsorption  equation  applies 
to  the  adsorption  of  ions  is  a  difficult  question,  into 
which  it  is  not  necessary  to  enter.  The  assumption 
that  different  ions  are  adsorbed  about  equally  is,  in 
any  event,  untenable,  as  is  evident  even  from  the 
small  selection  of  data  given  on  p.  56.  Quite 
recently  Wolfgang  Ostwald  (Koll.-Zeitschr.,  26,  28, 
69,  1920),  has  reviewed  a  very  large  mass  of  data  on 
electrolyte  coagulation,  and  has  come  to  the  conclu- 
sion that  Freundlich's  theory— while  selected  sets  of 
values  can  be  found  to  conform  to  it — in  a  large 
number  of  cases  fails  to  account  for  the  effects  of 
even  closely  related  electrolytes. 

These  criticisms,  of  course,  do  not  affect  the  theory 
which  ascribes  the  original  charge  to  adsorbed  ions. 
Whether  disperse  particles  would  be  electrically 
charged  in  a  medium  not  containing  ions,  or  whether 
stable  systems  of  colloidal  dispersity  are  possible, 


158          COLLOIDAL  ELECTROLYTES. 

in  which  the  disperse  phase  is  not  charged,  are 
questions  of  great  importance  in  this  connection,  to 
which  no  answer  is  possible  until  non-aqueous 
systems  have  been  studied  more  exhaustively  than 
has  been  the  case  so  far. 

We  must  now  consider  briefly  an  important  class 
in  which  the  charge  on  the  disperse  phase  need  not 
be  ascribed  to  adsorption  :  the  soaps  (p.  101)  and 
the  proteins  (p.  89)  are  its  most  important  repre- 
sentatives. The  behaviour  of  the  former  is  the 
simpler,  and  has  been  completely  elucidated  by  the 
investigations  of  McBain  and  his  school  (for  a  very 
lucid  summary  see  III.  British  Association  Report 
on  Colloid  Chemistry,  1920).  According  to  McBain, 
a  soap,  e.g.,  sodium  palmitate,  dissociates  into  a 
number  of  sodium  ions,  and  into  a  complex  or 
"  ionic  micelle  "  of  the  following  constitution  :— 

(NaP)*.(P>.(H20)m, 

i.e.,  a  hydrated  aggregate  of  undissociated  palmitate 
molecules  and  palmitate  ions,  the  number  of  the 
latter  being,  of  course,  equal  to  the  number  of  sodium 
ions.  The  whole  aggregate  behaves  as  an  anion, 
and  the  degree  of  hydration  varies  continuously 
with  concentration  and  temperature.  Its  size  is 
such  as  to  bring  it  within  the  colloidal  range  of 
dispersity. 

The  proteins  have  been  investigated  by  similar 
methods  by  Pauli  and  his  pupils,  and  the  results 
obtained  are  analogous.  As  we  have  seen  (p.  90), 
the  proteins  form  salts  with  either  acids  or  bases  ; 
the  former  dissociate  into  a  protein  cation,  and  one 
or  more  ordinary  anions,  while  the  latter  give  rise  to 
a  protein  anion  and  one  or  more  ordinary  cations. 
The  protein  ion,  whichever  its  charge,  is  complex, 
hydrated  and  colloidal. 

While  the  evidence  that  soaps  and  protein  salts 
behave  in  this  manner,  viz,,  as  "  colloidal  electro- 


COMPLEX  THEORY   OF  COLLOIDS.    159 

lytes,"  is  fairly  conclusive,  the  attempt  made  by 
Pauli  to  extend  the  theory  of  complex  formation 
to  all  colloidal  systems  is  likely  to  cause  controversy. 
The  attempt  is  based  on  careful  electrometric 
determinations  of  the  ion  concentration  in  various 
hydroxide  sols,  of  which  two  only  shall  be  mentioned 
here.  Ferric  hydroxide,  made  from  the  chloride,  is 
supposed  to  be  a  complex  salt  of  the  formula  :— 

*Fe(OH)3.;yFeCl3 

which  dissociates  into  the  complex  cation  : 
^Fe(OH)  3  .  yFe'  '  '  and  3yCl'.  An  analogous  structure 
is  suggested  for  stannic  acid  peptized  by  alkali  :— 

.  ySnO3K2, 


which  dissociates  into  the  complex  anion  :  #(SnO2)  . 
ySn03"  and  2yK  *  .  Finally  a  complex  structure  has 
been  ascribed  even  to  colloidal  gold  (General  Dis- 
cussion, Faraday  Soc.,  October,  1920).  In  the 
alkaline  reduction  of  gold  chloride  an  intermediate 
step  is  the  formation  of  an  aurate,  e.g.,  KO2Au,  and 
the  particle  is  looked  on  as  a  complex  of  metallic 
gold  and  aurate,  which  latter  dissociates  into  auric 
anion,  giving  the  particle  its  negative  charge,  and  a 
cation. 

These  views  have  only  recently  been  put  forward 
(although  an  earlier  complex  hypothesis  has  been 
formulated  by  Duclaux),  and,  while  likely  to  cause 
controversy,  are  here  mentioned  briefly  for  the  sake 
of  completeness.  Certain  difficulties  are  obvious 
in  the  case  of  metal  sols  ;  it  is  certainly  possible  to 
reduce  AuCl3  or  HAuCl4  without  alkali,  in  acid 
solution,  with  ethyl  alcohol  at  about  95°  C.,  when  the 
formation  of  an  aurate  appears  improbable  ;  equally 
so  is  the  existence  of  a  silver  compound  with  Ag  in 
the  anion,  which  would  be  necessary  to  explain  the 
negative  charge  on  the  particles  of  all  known  silver 
sols. 


160  ORIGIN  OF  CHARGE. 

The  reader  will  realize,  even  from  the  very  brief 
summary  given,  that  our  knowledge  of  the  origin  of 
the  charge  on  colloidal  particles  is  still  very  incom- 
plete. It  is,  of  course,  quite  possible  that  it  may  be 
produced  by  different  causes  in  different  systems, 
and  that  attempts  to  find  a  single  explanation 
applicable  to  all  cases  are  foredoomed  to  failure. 


CHAPTER   XXV. 

IT  now  remains  to  devote  a  few  words  to  applica- 
tions of  colloid  physics  and  chemistry,  both  theo- 
retical and  practical.  As  regards  the  former,  it  has 
given  the  impetus  to  the  study  of  the  Brownian 
movement  by  Svedberg,  Einstein,  v.  Smoluchowski 
and  Perrin,  which  has  afforded  the  most  striking  and 
convincing  proof  of  the  real  existence  of  molecules. 
No  such  conspicuous  success  has  yet  attended  the 
application  of  colloid  science  to  the  innumerable 
problems  of  physiology  calling  for  elucidation,  but 
this  is  a  field  full  of  the  greatest  promise.  Swelling 
in  water  and  in  solutions  of  electrolytes,  diffusion  in 
gels  and  through  membranes,  the  phenomenon  of 
syneresis,  the  deposition  of  insoluble  calcium  salts 
or  of  silica  in  gel-like  media,  all  have  an  important 
bearing  on  processes  occurring  constantly  in  living 
organisms.  Encouraging  starts  have  been  made  in 
many  directions  :  to  mention  one  or  two  examples 
only,  the  study  of  phase-reversal  in  emulsions  (p.  76) 
has  led  to  theories  of  protoplasmic  structure  and  of 
the  effect  of  "  antagonistic  ions  "  which,  while  in 
need  of  much  further  work,  open  a  new  way  of 
approaching  a  difficult  problem.  The  enhanced 
swelling  in  acid  has  been  studied  in  its  physiological 
aspect  by  Martin  Fischer  and  his  school,  who 
emphasize  its  bearing  on  fundamental  phenomena 
like  muscle  contraction  and  on  morbid  processes  like 
oedema. 

To  .take  another  branch  of  our  subject,  the  study 
of  adsorption  is  beginning  to  clear  up  a  great  number 
of  debated  questions  in  widely  different  fields. 


162  TECHNICAL   APPLICATION. 

Van  Bemmelen,  the  pioneer  in  this  line  of  research, 
was  led  into  it  through  studying  the  properties  of 
soils,  and  its  fundamental  importance  in  this  regard 
is  now  generally  recognized  (see  General  Discussion, 
Faraday  Soc.,  May,  1921).  At  the  other  end  of  the 
scale,  the  synthetic  preparation  of  the  photohaloids, 
i.e.,  coloured  adsorption  compounds  of  colloidal 
silver  and  a  silver  haloid,  has  solved  an  old  and 
famous  problem,  the  nature  of  the  latent  photo- 
graphic image. 

As  regards  practical  or  technical  applications,  the 
claims  of  colloidal  chemistry  are  perhaps  not  quite 
as  clear  yet,  and  no  useful  purpose  is  served  by 
exaggerating  them.  In  this  connection  it  may 
perhaps  be  well  to  remind  the  reader,  not  only  of 
the  youth  of  the  whole  discipline,  but  of  the  twofold 
way  in  which  the  development  of  a  branch  of  science 
may  bear  on  industries  and  arts.  It  may,  of  course, 
lead  directly  to  new  processes  and  manufactures  :  an 
instance  is  the  production  of  squirted  filaments  of 
the  refractory  metals  for  incandescent  lamps,  which 
were  made  from  the  finely  divided  metal  coagulated 
from  its  sol,  before  these  metals  could  be  drawn  into 
wire.  The  much  more  general  case — for  which  the 
growth  of  the  chemical  industries  and  of  chemistry 
provides  innumerable  illustrations — is  that  it  pro- 
vides explanations  of  phenomena  long  known  and 
dealt  with  empirically,  and  thus  opens  the  way 
to  further  progress.  In  this  direction  colloidal 
chemistry  finds  itself  in  a  position  of  particular 
difficulty,  inasmuch  as  many  of  the  industries  dealing 
with  typically  colloid  material,  such  as  ceramics, 
the  textile  industries,  the  fermentation  industries, 
tanning  and  some  branches  of  dyeing,  are  as  old  as 
history,  or  older,  and  have  attained  a  high  degree  of 
perfection  partly  empirically,  partly,  though  much 
later,  with  the  aid  of  chemistry.  In  all  of  them, 
however,  there  are  numerous  problems  which  have 


APPLICATIONS   OF  VISCOMETRY.        163 

so  far  withstood  attempts  at  solution  by  the  methods 
of  pure  chemistry.  The  number  and  importance  of 
such  problems  will  be  vividly  brought  home  to  the 
reader  of  the  three  British  Association  Reports  on 
Colloid  Chemistry.  For  these  questions  a  knowledge 
of  the  properties  of  colloids  provides,  if  rarely  an 
answer  at  the  first  approach,  at  least  an  entirely  new 
method  of  attack.  It  is  perhaps  natural  that  in 
some  cases  this  attack  should  take  the — not  very 
helpful — form  of  a  mere  restatement  of  the  problem 
in  the  terms  of  colloid  chemistry.  But  a  good  deal 
more  than  this  has  been  accomplished  and  even 
published,  and  it  is  not  surprising  that  industries 
of  more  recent  origin  should  have  shown  a  greater 
readiness  to  profit  by  new  knowledge.  Thus  the 
difficulties  arising  in  the  manufacture  of  margarine 
have  given  a  perceptible  impetus  to  the  study  of 
emulsions,  with  results  which,  one  may  hope,  have 
benefited  the  industry  and  are  certainly  an  addition 
to  our  scientific  knowledge.  The  increasing  use  of 
viscosity  measurements  in  very  different  fields  is 
another  step  in  the  right  direction.  One  of  the  first 
objects  of  scientific  control  must  necessarily  be  to 
find  some  quantitative  measure  or  parallel  of 
properties  which  are  ordinarily  judged  by  practice 
or  experience,  and  the  viscosity  of  sols  and  suspen- 
sions is  such  a  quantity,  which  stands  in  a  definite 
relation  to  qualities  as  difficult  to  define  as  the 
"  nerve  "  of  rubber  or  the  "  strength  "  of  flour. 
The  viscosity  of  rubber  sols  (p.  97)  and  the  remark- 
able parallelism  in  the  viscosities  of  sols  of  cellulose 
and  of  its  esters  (p.  94)  have  been  mentioned 
already.  The  viscosity  of  suspensions  of  flour  in 
water  has  been  investigated  by  Liiers  and  Ostwald 
(Koll.-Zeitschr.,  25,  82,  116,  1919)  who  find  that, 
other  things  being  equal,  flours  of  good  baking 
quality  give  considerably  higher  viscosities  than 
those  of  poor  quality.  Incidentally  other  problems 


164         SWELLING   AND   DISPERSITY. 

connected  with  the  properties  of  bread  have  been 
studied  from  the  point  of  view  of  colloid  chemistry 
by  these  authors,  by  Katz  and  others  (see  the  article 
by  R.  Whymper  in  the  Third  British  Association 
Report,  1920).  In  the  modern  industries  dealing 
with  cellulose  and  its  esters,  e.g.,  the  manufacture  of 
artificial  silk,  nitrocellulose  explosives  and  celluloid, 
viscosity  determinations  have  been  a  means  of 
control  probably  from  the  outset.  Similarly  some 
measure  of  the  adhesive  properties  of  glue  is  obtained 
by  determining  the  viscosity  of  the  sol  and  the  "  jelly 
strength,"  i.e.,  by  roughly  measuring  the  modulus  or 
elastic  limit  of  the  gel. 

While  the  phenomena  of  swelling  have  been  studied 
chiefly  in  their  biological  aspect,  work  on  the 
technical  side  has  not  been  wanting.  The  swelling 
of  starch  and  of  gluten,  and  the  further  changes 
undergone  by  their  gels  during  baking  and  ageing, 
have  received  consideration  in  the  investigations  on 
bread  which  have  already  been  quoted.  In  tanning 
swelling  plays  a  part  of  great  importance  and  this 
aspect  of  the  subject  has  been  studied  by  Procter 
and  his  school.  Problems  involving  the  degree  of 
dispersity  and  the  electric  condition  of  such  tanning 
agents  as  tannin  also  arise  in  this  industry  and  are 
beginning  to  receive  attention. 

The  effect  of  mere  changes  in  the  degree  of 
dispersity  on  the  physical  and  chemical  behaviour 
of  given  systems  has  been  known  empirically  in  many 
arts,  as  well  as  methods  of  controlling  it.  The 
ripening  of  photographic  emulsions,  e.g.,  involving  a 
decrease  in  the  dispersity  of  the  particles  of  silver 
haloid,  has  long  been  known  and  controlled  ;  the 
theoretical  aspect  of  the  processes  involved  is 
receiving  a  good  deal  of  study  since  the  methods  of 
colloid  chemistry  have  been  applied  to  the  problems 
involved.  The  connection  between  dispersity  and 
colour  is  of  importance  in  products  differing  as  widely 


ELECTRO-ENDOSMOSIS.  165 

as  glass  coloured  by  gold,  copper  or  selenium  on  the 
one  hand,  and  artificial  ultramarine  on  the  other, 
the  colour  of  which  is  now,  on  very  strong  grounds, 
ascribed  to  sulphur  in  a  highly  disperse  state. 
Every  chemist  is  familiar  with  the  methods  used  for 
making  certain  precipitates  less  disperse,  so  that  they 
can  be  retained  by  filtration,  and  the  rationale  of 
the  methods  used,  as  well  as  their  extension  to  fresh 
cases,  will  be  clear  to  any  one  who  has  studied  the 
formation  of  sols,  and  their  coagulation. 

Advantage  has  been  taken  of  the  electric  charge 
on  disperse  particles  in  other  directions.  Electro- 
endosmosis  and  cataphoresis  have  been  developed 
on  a  technical  scale  and  important  results  in  the 
purification  of  clays,  dehydration  of  peat,  etc.,  have 
been  claimed.  While  the  scientific  interest  of  these 
applications  is  considerable,  their  technical  success 
will  no  doubt  depend  on  purely  economic  factors, 
like  the  cost  of  electrical  energy  at  a  given  place. 
Cataphoresis  has  also  been  employed  for  the  separa- 
tion of  oil  from  oil-water  emulsions  (condensed 
water),  the  process  being  carried  out  between  iron 
electrodes  and  complicated  by  the  formation  of 
oxides. 

It  is  probably  unnecessary  to  extend  this  list,  as 
every  reader  engaged  in  technical  work  can  add  to  it 
problems  which  he  has  encountered.  Nor  will  he 
have  failed  to  notice  that  many  fundamental  points 
in  theory  still  require  elucidation.  In  both  directions 
there  is  an  unlimited  field  for  work,  and  to  the 
student  this  should,  perhaps,  be  not  the  least  attrac- 
tive feature  of  a  fascinating  branch  of  physical 
science. 


SUBJECT-MATTER   INDEX. 


A. 

PAGE 

Adsorption      .......  -136 

and  chemical  changes       .          .          .          .          .146 

anomalous      .....  .    145 

compounds     .......    108 

electrical         .          .          .          .          .          .  -152 

equilibrium    .  .  .  .  .  .  .140 

isotherm         .          .          .          .          .  .          .142 

negative          .          .          .          .  .          .  -136 

of  dissociated  substances  .          .          .  .146 

selective          .          .          .          .  .  .          .148 

Agar  gels        ........      105,  117 

sols 81 

Albumin  sols  ........      86 

and  Hofmeister  series  .          .          .          .          .88 

electrolyte  coagulation  of  .  .         87,  88 

heat  coagulation  of  .  .  87 

highly  purified      .  .      go 

Aluminium  hydroxide  (for  adsorption)        .  .          .          .    139 

sol 66 

Amicrons         .....  .21 

Arsenic  trisulphide  sol      .          .          .          .          .          .          -43 


B. 
Brownian  movement         .......      33 


Cadmium  sulphide  sol  •      45 

Camphorylphenylthiosemicarbazide  gel        .  .129 

Ceric  hydroxide' sol  •      68 

Congo  Red,  osmotic  pressure  of  .102 

size  of  molecule       .  •      J3 

Continuous  phase    ...  -7 

Crystalloids     .  I 


SUBJECT-MATTER    INDEX.  167 

D. 

PAGE 

Dialysis  .  .  .  .  .  .  .  .     i,  TO,  13 

Diffusion  in  gels       .  .  .  .  .  .  .  .118 

Disperse  phase          ........        7 

Disperse  system       ........        7 

possible  types  of  .          .  .  .8 


Electric  charge  influence  of  reaction  of  medium  .  .    155 

methods  of  demonstration             .  .  38,  39 

on  disperse  phase       .           .          .  .  -37 

theories  of  origin  of   .           .           .  .  .    154 

Electrolytes,  action  on  emulsoids        .           .           .  .  .100 

action  on  suspensoids    .           .           .  .  -54 

effect  of  valency  .           .           .           .  .  -55 

Freundlich's  theory  of  action            .  .  .    156 

Hofmeister  series  of                  .           .  .  .88 

quantities  taken  down  by  gels          .  .  -55 

specific  effects  of  ions    ....  56,  57 

velocity  of  coagulation            .          .  .  -59 

Emulsifying  agents            .          .          .          .          .  .  •      7- 

effect  on  interfacial  tension  .  .  -73 
Emulsions       .........      70 

pure  oil-water            .           .           .           .  .  71 

reversal  of  phases  in           .           .           .  .  -75 

rich  in  disperse  phase         .           .           .  .  72 

viscosity  of                 .           .           .           .  .  .76 

Emulsoids       .           .           .           .           .           .           .  •  3°.  77 

physical  properties  of                    .           .  .  -94 
ultra-microscopic  image  of          ....      95 

Exponents  of  concentration  function            .           .  .  .142 

Extraction  curves    .          .          .          .          .          .  .  -151 


Ferric  hydroxide  sol          ......          66,  68 


rf-Galactan      .  .  .  .  .  .  .  .  .81 

Gelatin  sols     .....  .  .      81 

Gels        .          .          .  .  .  .  .  .          .  .105 

diffusion  in  .  .  .  .  .  .  .118 

elastic      ........      105,  in 

reactions  in       .  .  .  .  .  .  .  .119 

rigid         .  ....    105 

Gold  numbers  ........      63 

sols          .  .  .  .  .  .  .  .       3,  5,  40 


168  SUBJECT-MATTER    INDEX. 

H. 

FACE 

Hysteresis  (of  setting  temperature)    .          .  .          .          .82 

I. 
India-rubber  sols,  viscosity  of  .  .  .          .          .          -97 

L. 

Liesegang's  phenomenon  .          .          .          .          .  .122 

Lyophile          .........      25 

Lyophobe        .........      25 

Lyotropic        .........      89 

M. 

Mercury  sols  .........      42 

Mercuric  sulphide  sols       .......      45 

Methods  of  preparing  suspensoid  sols  .  .  .  .40 

Microns  2 1 


Night  Blue,  adsorption  by  sand           .                                 .           .  152 

anomalous  adsorption  of           .           .                      .  145 

O. 

Oscillating  discharge          .......  46 

Osmotic  pressure  of  sols  .......  36 


Peptization     .  .  .  .  .  .  .  .  -45 

Platinum  sols  ........      42 

Precipitation,  mutual,  of  sols    .  .  .  .  .  .62 

Protective  colloids  ......  -63 

S. 

Silicic  acid  gel           .          .          .          .          .          .          .  .106 

dehydration  of  .           .           .           .           .  .    107 

sol    .          .          .          .                    .          .  .78 

structure  of  dry  .  .  .  .  .109 

Silver  sols       .                               .....  3,  41 

.  16,  17,  21 

4°>  44 
10 


Size  of  particles 
Sols,  formation  of 
Specific  surface 
Stannic  acid  sol 


67 


SUBJECT-MATTER   INDEX.  169 


PAGE 

Surface  energy         .... 

.     132 

tension         .... 

.     132 

Suspended  particle,  motion  ol  . 

.        31 

Suspensions    ..... 

.        8 

Suspensoids    ..... 
Swelling  of  elastic  gels 

30,  40 
.    in 

heat  of 

....     112 

rate  of 

.     II4 

T. 

Tungstic  acid  sol      . 

.       80 

Tyndall  cone  ..... 

.      16 

U. 

Ultra-condenser       .... 

.        22 

Ultra-microscope     .... 

.        19 

V. 
Velocity  of  coagulation     ...  59 


NAME   INDEX. 


A. 


Amberger,  64 
Ambronn,  17 
Anderson,  109 
Auerbach,  52 


B. 

Bachmann,  109 
Bancroft,  75 
Bayliss,  98,  102 
Bechhold,  14,  15,  65,  119 
Bemmelen,  van,  106,  162 
Benson,  135 
Berzelius,  2,  43,  78 
Bhatnagar,  76 
Biltz,  62,  145 
Bradford,  124,  129 
Bredig,  46 
Brown,  33 
Burton,  37 
Biitschli,  127 


C. 


Chick,  87,  95 
Clowes,  75 
Coehn,  155 
Cotton,  39 


I). 


Davis,  152 
Debus,  3 
Debye,  28 


Donnan,  71,  73 
Dreaper,  152 


Einstein,  34,  35,  49 
Ellis,  71 
Evelyn,  3 
Exner,  33 


F. 

Faraday,  4,  63 
Fernau,  68 
Fizeau,  17 

Freundlich,  5,  25,  54,  56,  57, 
89,  140,  141,  156 


G. 

Garrett,  29,  98 
Gibbs,  136 
Goodwin,  71 
Goppelsroeder,  148 
Gouy,  33 

Graham,  i,   5,   45,   66,  78,  So, 
5,  106,  117,  118 


105, 


H. 


Hardy,  4,  37,  54,  55 
Hatschek,  15,  29,  49,  71,  83, 

98,  124,  129 
Helmholtz,  154 
Hess,  50 


NAME  INDEX. 


171 


Holmes,  120 
Hofmeister,  86,  88 
Humphrey,  49 


Jevons,  33 


J- 


K. 


Katz,  126,  164 
Keen,  52 
Kohlschutter,  41 
Kraus,  148 
Krecke,  68 
Kuzel,  45 

L. 

Lamb,  154 

Lea,  Carey,  3,  41 

Leick,  115 

Lewis,  71 

Liesegang,  121 

Lillie,  98 

Linder,  4,  43,  54,  62 

Lottermoser,  43,  155 

Liiers,  51,  163 


M. 

McBain,  101,  128,  141,  158 
Martin,  14,  87,  95 
Michaelis,  143 
Moore,  98 
Mouton,  39 
Miiller,  66,  68 
Muthmann,  3 


Naegeli,  128 


N. 


o. 


Oden,  42,  48,  58 
Oryng,  146 


Ostwald,  Wilheim,  i,  123,  140 
Ostwald,  Wolfgang,  5,  7,  25, 
65,  94.  I02>  I17>  J57 


P. 

Paal,  64 

Paine,  59 

Pauli,  30,  68,  82,  84,  90,  126, 

158 

Perrin,  25,  35,  36 
Pickering,  72,  75 
Picton,  4,  43,  54,  62 
Porter,  52 
Powis,  155 
Pringsheim,  120 
Procter,  126,  164 
Punter,  94 


Quincke,  127 


R. 


Raffo,  42 
Ramsay,  33 
Ramsden,  135 
Rayleigh,  16 
Reinke,  113 
Roaf,  98 
Rona,  30,  143 


S. 


Scherrer,  28,  109 
Schidrowitz,  97 
Schlaepfer,  75 
Schroeder,  in 
Schultze,  55 
Selmi,  3 

Siedentopf,  4,  17 
Smoluchowski,  34,  35,  60,  154 
Sobrero,  3 
Soddy,  36 


1 72 


NAME    INDEX. 


Spence,  150 

Stokes,  31 

Svedberg,  5,  34,  40,  46 


Tyndall.  16 


T. 


W. 


Wackenroder,  3 
Walpole,  85 


Weimarn,  v.,  5,  44,  93,  129 
Whetham,  55 
Wiedemann,  112 
Wiegner,  137 
Wiener,  33 
Wislicenus,  139 
Wohler,  3 


Z. 


Ziegler,  119 

Zsigmondy,  4,  5,   17,   33,   60, 
63,  65,  67,  109 


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